AMDGPU: Improve documentation.

Summary:
Add links to ISA manuals and ABI.
Add text about assembler syntax.
Add info about instructions operands.
Add instruction examples for each encoding.
Update directives section, add missing .amdgpu_hsa_kernel.

Reviewers: tstellarAMD, SamWot, vpykhtin

Subscribers: kzhuravl, wdng, nhaehnle, yaxunl, tony-tye, artem.tamazov, llvm-commits

Differential Revision: https://reviews.llvm.org/D24724

llvm-svn: 281962