Small doc tweak for array/string functions

Hi

On these pages:

- https://www.postgresql.org/docs/current/functions-array.html
- https://www.postgresql.org/docs/current/functions-string.html

we point out via "See also" the existence of aggregate array and string
functions, but I think it would be useful to also mention the existence
of string-related array functions and array-related string (regexp) functions
respectively.

(Background: due to brain fade I was looking on the array functions page
for the array-related function whose name was escaping me which does something
with regexes to make an array, and was puzzled to find no reference on that page).

Regards

Ian Barwick

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Ian Barwick http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services