Improve psql \?'s description of large-object-related commands.

Improve psql \?'s description of large-object-related commands.

Provide a gloss of which command does what, as all other backslash
commands have.  Put the large-object command section into a more
considered spot in the list.

In passing, update the output-lines count in helpVariables()
(oversight in 7844c9918, looks like).

Thibaud Walkowiak, reviewed by Nathan Bossart and myself

Discussion: https://postgr.es/m/43f0439c-df3e-a045-ac99-af33523cc2d4@dalibo.com