pgsql: Rework word_similarity documentation,make it close to actual al

Rework word_similarity documentation, make it close to actual algorithm.

word_similarity before claimed as returning similarity of closest word in
string, but, actually it returns similarity of substring. Also fix mistyped
comments.

Author: Alexander Korotkov
Review by: David Steele, Liudmila Mantrova
Discussionis:

https://www.postgresql.org/message-id/flat/CY4PR17MB13207ED8310F847CF117EED0D85A0@CY4PR17MB1320.namprd17.prod.outlook.com
https://www.postgresql.org/message-id/flat/f43b242d-000c-f4c8-cb8b-d37e9752cd93%40postgrespro.ru

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/5b1b7286c9e12dfe6aad6d722b704d3e948a3d03

Modified Files
--------------
contrib/pg_trgm/trgm_op.c |  4 ++--
doc/src/sgml/pgtrgm.sgml  | 56 +++++++++++++++++++++++++++++++++++------------
2 files changed, 44 insertions(+), 16 deletions(-)