API: Warn about POST without Content-Type

HHVM sniffs the content type and so probably correctly guesses
application/x-www-form-urlencoded and fills in $_POST, while PHP 7 does
not sniff and so doesn't fill in $_POST.

Since there are clients that have been expecting the HHVM behavior, give
a warning both to let them know and so we can have some idea of how many
such clients there are.

This also adds any warnings and errors as an HTML comment at the top of
the auto-generated help page, to hopefully make it easier for people
getting that unexpectedly to find out why.

Bug: T230735
Change-Id: I017b7afe808844d74d376f6436894a5a2f525a9f