LoongArch: Document -mexplicit-relocs={auto,none,always}

gcc/ChangeLog:

	* doc/invoke.texi (-mexplicit-relocs=style): Document.
	(-mexplicit-relocs): Document as an alias of
	-mexplicit-relocs=always.
	(-mno-explicit-relocs): Document as an alias of
	-mexplicit-relocs=none.
	(-mcmodel=extreme): Mention -mexplicit-relocs=always instead of
	-mexplicit-relocs.