Cups 3.x documentation WriteRequest and Error functions

[RolandHughes]

This section of the documentation needs to state a single file or data stream may be broken up into multiple small writes to conserve resources. It also mentions PPD which is supposed to have been taken out and shot.

While I'm being a documentation PITA, cupsLastError() and cupsLastErrorString() are mentioned all over in the documentation, but they have no links on the left and the internal links don't work.

https://openprinting.github.io/cups/libcups/cupspm.html#cupsLastErrorString

Originally posted by @RolandHughes in #98 (comment)

[michaelrsweet]

[master 0d2f5cd] Update documentation (Issue #99)