Clarify instuctions for using Review Board with Subversion.
Clarify instuctions for using Review Board with Subversion.

Updated Linux installation documentation for Subversion to indicate that PySVN
is preferred over subvertpy.

Testing Done:
Built docs.

Reviewed at https://reviews.reviewboard.org/r/7832/

reviewboard/reviewboard,davidt/reviewboard,sgallagher/reviewboard,chipx86/reviewboard,davidt/reviewboard,brennie/reviewboard,sgallagher/reviewboard,brennie/reviewboard,sgallagher/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,brennie/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,chipx86/reviewboard,brennie/reviewboard,reviewboard/reviewboard,davidt/reviewboard,davidt/reviewboard