The wording is obscure, but the advice is to modify the man.local file itself rather than override macros found in that file.
A good place to start is with the documentation: 5.21 Writing Macros (The GNU Troff Manual). That shows how to define macros.
The actual macros to study are heavily version-dependent. In Debian, I find the PT macro in the groff-base package, in
/usr/share/groff/1.21/tmac/an-old.tmac
as
.\" Redefine these to customize the header & footer
.
.de1 PT
. tl '\\*[an-title](\\*[an-section])'\\*[an-extra3]'\\*[an-title](\\*[an-section])'
..
It passes named variables as the first parameter to .tl. To see what .tl is (grep is your friend) and what it does takes some work (mentioned here):
Are you aware that .tl (title) takes three parts, left, centre, and
right?
On the other hand, groff's documentation has improved over the years: you can find ".tl" in the index.
As a rule, you will find that:
- most of the macros (which you encounter in reading these files) are undocumented
- grep'ing over the list of macros (grep
-w helps) is the only way to find a particular macro
- there are device-specific versions of some macros which override generic versions
