fix and result

It is not clear in http://pfultz2.github.io/Fit/doc/html/fix/index.html on which cases it is needed to use fit::result.

> "In addition, due to the eagerness of constexpr to instantiation templates, **in some cases**, an explicit
> return type must be specified in order to avoid reaching the recursion limits of the compiler. "

Open the linked fix documentation and locate the section containing the quoted explanation of fit::result. Read the surrounding examples and determine which cases require an explicit result type; update the wording so those cases are clear to users. Done means the documentation directly explains when fit::result is needed.

Written by the indexing model from the issue text.