Talk about URL support and the real meaning of `-` in the latest `fromSrc` API documentation.

Signed-off-by: Jean-Paul Calderone <exarkun@twistedmatrix.com>
Update the most recent released version of the docs as well.

Signed-off-by: Jean-Paul Calderone <exarkun@twistedmatrix.com>
Merge pull request #9486 from ClusterHQ/9484-fromSrc-api-docs

Document fromSrc more clearly