Parameterized Request content file

[exarkun]

Contributor Checklist:

• There is an associated ticket in Trac. Create a new one at https://twistedmatrix.com/trac/ticket/9655
• The changes pass minimal style checks (see: https://twistedmatrix.com/trac/wiki/TwistedDevelopment#GettingYourPatchAccepted )
• I have created a newsfragment in src/twisted/newsfragments/ (see: https://twistedmatrix.com/trac/wiki/ReviewProcess#Newsfiles )
• I have updated the automated tests.

[exarkun]

This would probably benefit from some documentation but the state of Twisted Web documentation makes it very difficult to see where such information would fit in.

[exarkun]

The circleci/documentation failure is a regression in the released-today docutils 0.15. Merging.