Docs: tidy up various usages of the productname tag

Docs: tidy up various usages of the productname tag

This tries to bring a bit more consistency to the use of the <productname>
tag in the documents.  This fixes a couple of mistakes with our own
product.  We definitely should be consistently calling that PostgreSQL
when we're referring to the modern-day version of the software.

This also tidies up a couple of inconsistencies with the case of other
product names, namely Emacs and Python.  We also get rid of some incorrect
usages of <productname> and replace them with <literal>.

Many of these mistakes exist in the back branches, but they don't quite
seem critical enough to warrant fixing them in prior versions at this
stage.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20220411020336.GB26620@telsasoft.com