internal/typeparams: improve the package documentation
internal/typeparams: improve the package documentation

Update and expand on the package documentation for the
internal/typeparams package.

Change-Id: I2af16250580abf9e6b3a8692356faa498ba83f29
Reviewed-on: https://go-review.googlesource.com/c/tools/+/377654
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Reviewed-by: Tim King <ee8a1d1efb05d8171fc0140ced15d455e83941c5@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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