The loops to identify word boundaries could access past the end of
the input string.  Likely that would never result in an actual
crash, but it makes valgrind unhappy.

The logic to try different numbers of words didn't work when the
input has two words but we only have a match to the first, eg
"\h with select".  (We must "continue" the pass loop, not "break".)

The logic to compute nl_count was bizarrely managed, and in at
least two code paths could end up calling PageOutput with
nl_count = 0, resulting in failing to paginate output that should
have been fed to the pager.  Also, in v12 and up, the nl_count
calculation hadn't been updated to account for the addition of a URL.

The PQExpBuffer holding the command syntax details wasn't freed,
resulting in a session-lifespan memory leak.

While here, improve some comments, choose a more descriptive name
for a variable, fix inconsistent datatype choice for another variable.

Per bug #16837 from Alexander Lakhin.  This code is very old,
so back-patch to all supported branches.

Kyotaro Horiguchi and Tom Lane

Discussion: https://postgr.es/m/16837-479bcd56040c71b3@postgresql.org