manual: improve string section wording

* manual/string.texi: Editorial fixes.  Do not say “text” when
“string” or “string contents” is meant, as a C string can contain
bytes that are not valid text in the current encoding.
When warning about strcat efficiency, warn similarly about strncat
and wcscat.  “coping” → “copying”.
Mention at the start of the two problematic sections that problems
are discussed at section end.