Made many minor changes so that the document is consistent with itself.

Some more noticeable changes:
* CL/34141: Revert "testing: add T.Context method"
* CL/33630: net/http: document restrictions on ETag as expected by ServeContent

Change-Id: I39ae5e55c56e374895c115e6852998c940beae35
Reviewed-on: https://go-review.googlesource.com/34243Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>