gdb/doc: fix help text for 'set style disassembler enabled'

gdb/doc: fix help text for 'set style disassembler enabled'

The help text for 'set/show style disassembler enable' was output of
date.  It talks about GDB requiring the Python Pygments library, but
for many common architectures this is no longer the case, libopcode is
used for styling.

The Python Pygments library is still used as a fallback for those
architectures that libopcode doesn't currently style.

I've updated the help text to try and explain all this.  The manual
was already updated.

Approved-By: Eli Zaretskii <eliz@gnu.org>