[DOCS] Consistent capitalization of procedural languages

In a not-so-rare case of OCD I realized we have a few different ways to
capitalize the names of procedural languages in the documentation (and code
comments).  There is one way used almost exclusively, and then a few less
common variants:

PL/pgSQL: 867
PL/pgsql: 22
PL/PgSQL: 15
pl/pgsql: 8
PL/pgSqL: 1
Pl/pgSQL: 1

PL/Perl: 618
PL/perl: 10
pl/perl: 6

PL/Python: 781
PL/python: 15
pl/python: 6

PL/Tcl: 289
PL/TCL: 2
pl/tcl: 1

The attached patch moves to consistently using PL/pgSQL, PL/Perl, PL/Python and
PL/Tcl.  Language files are left as-is, but if this is applied I’m happy to fix
those up as well and submit to -translators.

cheers ./daniel