[nvptx, doc] Update misa and mptx, add march and march-map

[nvptx, doc] Update misa and mptx, add march and march-map

Update nvptx documentation:
- Use meaningful terms: "PTX ISA target architecture" and "PTX ISA version".
- Remove invalid claim that "ISA strings must be lower-case".
- Add missing sm_xx entries.
- Fix misa default.
- Add march, copying misa doc.
- Declare misa an march alias.
- Add march-map.
- Fix "for given the specified" typo.

gcc/ChangeLog:

2022-03-29  Tom de Vries  <tdevries@suse.de>

	* doc/invoke.texi (misa, mptx): Update.
	(march, march-map): Add.