Remove documentation on installing nightlies.

Remove documentation on installing nightlies from the "Installing Development
Releases" section of the manual, since nightlies are no longer built. Also
clarify the bit about differentiating from the installation docs, meaning
standard (production) installation docs.

Testing Done:
Visually inspected generated HTML docs.

Bugs closed: 3252

Reviewed at https://reviews.reviewboard.org/r/5514/