Grammar fix in the docs.
Document that attributes can be changed in the helper methods.
Document the auto_pagination default in Pithub::Base.

It's documented in Pithub::Result, but docs should be where the user
is and every other attribute in Pithub::Base documents its default here.
Document the per_page maximum.

Defined as the maximum allowed by Github, not as 100.  This will allow us
to change the default later if Github allows more and have plausible
deny-ability if people relied on it being 100.

Document why per_page is set to 100.

Explain what per_page does.

Link to auto_pagination which is important to go with per_page.