GUC description cleanup

This patch makes the short_desc and extra_desc fields of the GUC table
more consistent: both text in both fields should be complete sentences
that begin with a capital letter and end in a period.

Note that this patch breaks the translations of these strings, so I
haven't applied it yet. Should I apply it now, or wait for 8.3 to
branch?

BTW, should pre_auth_delay be included in SHOW ALL? Its short_desc is
currently "No description available", so I think we should either add a
proper description or omit pre_auth_delay from SHOW ALL.

-Neil