Add a funstions category and resolve some sphinx warnings

[Nick Draper]

We need a category for Functions, as it is called in Fit_v1 many times.

Also this ticket will try to resolve warnings on a set of classes.

It is not intended to fix all sphinx warnings, in fact the number resolved will not be dramatic.

[Nick Draper]

re #9649 A few warnings, but more importanly a catergory for functions

Changeset: 820ad406391d8c0b150dd8602ffb9f77fa771508

[Nick Draper]

re #9649 Another couple of warnings

Changeset: d4ca7f8b82e6e4acdf7dd4ce06fc508adb994cad

[Nick Draper]

To test:

1. build the docs
2. check the fit algorithm docs, check that the links to the fit functions category work

   One is in this text
     A list of the available simple functions can be found here.
   

3. This should also resolve a few sphinx warnings, check there is a net decrease.

[Owen Arnold]

Merge remote-tracking branch 'origin/feature/9649_Sphinx_warnings'

Full changeset: ​b604f0e630687a1f990cf383e8588aced8918348

[Stuart Campbell]

This ticket has been transferred to github issue ​10492