C-h f insert RET

   insert is a built-in function in ‘src/editfns.c’.

   [...]

   When operating on binary data, it may be necessary to preserve the
   original bytes of a unibyte string when inserting it into a multibyte
   buffer; to accomplish this, apply ‘string-as-multibyte’ to the string
   and insert the result.

C-h f string-as-multibyte RET

   string-as-multibyte is a built-in function in ‘src/fns.c’.

   This function is obsolete since 26.1; use `decode-coding-string'.

   [...]

This looks inconsistent, presumably 'insert' shouldn't mention
'string-as-multibyte'. Attached patch updates the doc string.

Also, the elisp manual still mentions functions string-as-unibyte and
string-as-multibyte in section "(elisp) Top > Non-ASCII Characters >
Selecting a Representation". Should these be removed from the manual,
or at least be labelled as obsolete?