Errors in explanation of the --with-libxml configure option.

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/12/install-procedure.html
Description:

Online doc states

        [...], or set the variables XML2_CFLAGS and XML2_LIBS. (If
pkg-config is installed, then to override its idea of where libxml2 is you
must either set XML2_CONFIG or set both XML2_CFLAGS and XML2_LIBS to
nonempty strings.)

But neither XML2_CFLAGS nor XML2_LIBS appears in the actual configure
scripts or makefiles, and so of course setting those env vars does no good.
The INSTALL text file bundled in postgresql-12.1.tar.bz2 states instead

        [...] , you can either set the environment variable XML2_CONFIG to
point to the "xml2-config" program belonging to the installation, or use the
options "--with-includes" and "--with-libraries".

Now *that* advice is correct, and following it leads to success.  Therefore
suggest sync that part of online doc to corresponding part of INSTALL
offline doc.

Thank you.