Doc: add cross-references between array_to_string and string_to_array.

Doc: add cross-references between array_to_string and string_to_array.

These functions aren't exact inverses, but they're closely related;
yet we document them in two different sections.  Add cross-ref
<link>s to improve that situation.

While here, move the strpos and substr entries to re-alphabetize
Table 9.10.  Also, drop an ancient compatibility note about
string_to_array, which wasn't even on the right page, so probably
few people ever saw it.

Discussion: https://postgr.es/m/164287017550.704.5840412183184961677@wrigleys.postgresql.org