gopls/doc: update and expand the nvim documentation
gopls/doc: update and expand the nvim documentation

Change-Id: If7df2c5b0cefb73e96641187bcd721962d72cb4d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/257437
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Trust: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

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