9482 httpfactory docstring

[twm]

Document some undocumented parameters to twisted.web.http.HTTPFactory.

Contributor Checklist:

• There is an associated ticket in Trac. Create a new one at https://twistedmatrix.com/trac/ticket/9482
• 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. (N/A)

[codecov]

Codecov Report

Merging #1037 into trunk will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            trunk    #1037     +/-   ##
=========================================
- Coverage   91.84%   91.75%   -0.1%     
=========================================
  Files         844      844             
  Lines      150593   150986    +393     
  Branches    13141    13234     +93     
=========================================
+ Hits       138312   138537    +225     
- Misses      10181    10360    +179     
+ Partials     2100     2089     -11

[glyph]

This looks like a great documentation enhancement. Please merge at your convenience!