Doc: improve documentation for ^@ starts-with operator.

Doc: improve documentation for ^@ starts-with operator.

This operator wasn't formally documented anywhere.  To give it
a natural home, relabel the functions-string-other table as
"Other String Functions and Operators", which is more parallel
to the functions-string-sql table anyway.

While here, add cross-references to the pattern match and text
search sections.  It seems moderately likely that people would
come to this section looking for those (but I don't want to
actually list them in these tables).

Discussion: https://postgr.es/m/CADT4RqB13KQHOJqqQ+WXmYtJrukS2UiFdtfTvT-XA3qYLyB6Cw@mail.gmail.com