go/doc: remove brackets from receiver in example identifier
When constructing a string for a method that will match an example
function's name, remove brackets from the receiver. This makes it
possible to write an example associated with a method of a generic
type.
Also, modify the test for classifying examples to check that all the
expected examples actually appear.
Fixes golang/go#52496.
Change-Id: Iebc5768f6cb91df9671dd701b97958fb8081f986
Reviewed-on: https://go-review.googlesource.com/c/go/+/401761
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com> Reviewed-by: Robert Findley <rfindley@google.com>