For two functions available in Basic, namely Replace and StrConv, there is no help. These functions (as far as I understand, they were modelled upon eponymous functions in VBA) were introduced into OpenOffice.org and obviously also into LibreOffice (see https://bz.apache.org/ooo/show_bug.cgi?id=100953) as tentative, but they are still active, and at least the first one seems to be working completely. So, as some people already suggested, there should be a description in the help files, for they supply helpful functionality which would otherwise have to be programmed.
I supply descriptions that I hope can be used for the help files. In the first case I think it is complete, in the second case I provide a text which only covers the cases I could test, and a separate description of the rest where I am not sure what is happening.
For the second case, the function StrConv, there are could not get a convincing result regarding conversion from/to Unicode; hence I propose a text which may well be adjusted .
Especially, I am not sure whether a help text for StrConv should be published, as some functionality could not be verified by me, and the third parameter seems to be still irrelevant. Andrew Pitonyak provided some information in his OOME book, which could not be confirmed in the case of Unicode conversion, a situation which may well be outdated.


Replace Function
Replaces some string by another.

Syntax:
Replace (Text As String, SearchStr As String, ReplStr As String [, Start As Long [, Count as long [, Compare As Boolean]]]

Return Value:
String

Parameters:
Text: Any string expression that you want to modify.
SearchStr: Any string expression that shall be searched for.
ReplStr: Any string expression that shall replace the found search string.
Start: Numeric expression that indicates the character position within the string where the search shall begin. The maximum allowed value is 65535.
Count: The maximal number of times the replace shall be performed.
Compare: Optional numeric expression that defines the type of comparison. The value of this parameter can be 0 or 1. The default value of 1 specifies a text comparison that is not case-sensitive. The value of 0 specifies a binary comparison that is case-sensitive. You can as well use FALSE instead of 0 or TRUE instead of 1.

Error codes:
5 Invalid procedure call

Example:

msgbox replace ("aBbcnnbnn", "b", "$", 1, 1, FALSE)	'returns "aB$cnnbnn"
meaning: "b" should be replaced, but
    • only when lowercase (parameter 6), hence second occurrence of "b"
    • only first (respecting case) occurrence (parameter 5)


StrConvFunction
Converts a string according to the parameters supplied.

Syntax:
StrConv (Text As String, Mode As Long[, locale_id As Long ])

Return Value:
String

Parameters:
Text: Any string expression that you want to modify.
Mode: An integer (Long) value describing the type of conversion you want to be done:
0	No change.
1	Convert all characters to uppercase.
2	Convert all characters to lowercase.
3	Convert first characters of each word to uppercase.
4	Convert narrow (half-width) characters in the string to wide (full-width) characters.
8	Convert wide (full-width) characters in the string to narrow (half-width) characters.
16	Convert Hiragana characters in the string to Katakana characters.
32	Convert Katakana characters in the string to Hiragana characters.
Modes 4 and 8 are mainly applicable to Far East character sets; Modes 16 and 32 are used for Japanese character sets

Error codes:
5 Invalid procedure call

Example:

msgbox StrConv("abb jkk",3)	'returns "Abb Jkk"


Additional information for StrConv, to be added in some form to the above help only if found to be correct:
The description for the following values of parameter “mode” as found in Andrew Pitonyak’s OOME is not very clear: 
64	Convert all characters to Unicode.
128	Convert all characters from Unicode.
The following explanation is derived from Microsoft’s function:
64	Converts the string to Unicode using the default code page of the system.
128	Converts the string from Unicode to the default code page of the system.
The output of a call with mode 128 is for my system an array of bytes, and according to information found somewhere in the Microsoft files, it should be an array of multiple bytes for other codepages, e.g. asian ones. Such an array can be used as input for a call with mode 64. It seems to me that the description derived from the Microsoft function applies to the LibreOffice function, but then I could only test it with the codepage used in Germany (which is probably 1252). I am not able to test these modes in other situations.
One reason not to mention the modes 64 and 128 might also be that there may possibly not much need now for them, as Unicode is prevalent.