Doc: various improvements for pg_basebackup reference page.

 Doc: various improvements for pg_basebackup reference page.
 
 Put the -r option in the right section (it certainly isn't an
 option controlling "the location and format of the output").
 
 Clarify the behavior of the tablespace and waldir options
 (that part per gripe from robert@interactive.co.uk).
 
 Make a large number of small copy-editing fixes in text that
 visibly wasn't written by native speakers, and try to avoid
 grammatical inconsistencies between the descriptions of
 the different options.
 
 Back-patch to v13, since HEAD hasn't meaningfully diverged yet.
 
 Discussion: https://postgr.es/m/159749418850.14322.216503677134569752@wrigleys.postgresql.org