gopls/doc: Document how gopls generates semantic tokens.
gopls/doc: Document how gopls generates semantic tokens.

The new file gopls/doc/semantictokens.md describes how gopls presently
decides which semantic tokens and modifiers to send to clients.

Change-Id: Ifc76e13954dde2ea58b6da446517f94be038a285
Reviewed-on: https://go-review.googlesource.com/c/tools/+/333010
Run-TryBot: Peter Weinberger <dd140fb9530d791057dd8833ea6768ff65fcde71@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Trust: Peter Weinberger <dd140fb9530d791057dd8833ea6768ff65fcde71@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools