Fix collation of expressions in GRAPH_TABLE COLUMNS clause

Fix collation of expressions in GRAPH_TABLE COLUMNS clause

GRAPH_TABLE clause is converted into a rangetable entry, which is
ignored by assign_query_collations().  Hence we assign collations
while transforming its parts.  But expressions in COLUMNS clause
missed that treatment, so fix that.

While at it, also add comments about collation assignment to the parts
of GRAPH_TABLE clause, and also fix a small grammar issue.

Reported-by: Satyanarayana Narlapuram <satyanarlapuram@gmail.com>
Author: Satyanarayana Narlapuram <satyanarlapuram@gmail.com>
Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://www.postgresql.org/message-id/CAHg+QDc4aaiufYSgrwMMPMMRTPtQ66SghcrPFbWJFZMqNaG+BA@mail.gmail.com