CWG2965 [basic.scope.temp] Add a note that a lambda doesn't have a template parameter scope

[t3nsor]

Full name of submitter: Brian Bi

Issue description: It is extremely counterintuitive that a generic lambda doesn't have its own template parameter scope. This should be explicitly called out.

Suggested resolution: Add a note at the end of [basic.scope.temp]/2:

[Note: The optional template-parameter-list of a lambda-expression does not introduce a template parameter scope. — end note]

The description of [CWG2838] should also be amended; it currently claims that there is a template parameter scope there.

[jensmaurer]

CWG2965

Description of CWG2838 was fixed.