pgsql: Doc: add cross-references between array_to_string and string_to_

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4f02cbcb6839553c595efa3a2eb1c2a09557ef79

Modified Files
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doc/src/sgml/func.sgml | 114 ++++++++++++++++++++++---------------------------
1 file changed, 52 insertions(+), 62 deletions(-)