[sf.cmath.assoc.laguerre,sf.cmath.assoc.legendre] Add reference to eq

[mattkretz]

The associated Laguerre/Legendre functions build on the Laguerre/Legendre functions, which are defined in different equations. Point to them from the associated functions.

Also use the correct \ell as used in the formula.

[mattkretz]

Oh, there are more of such formula reuses. The other functions typically use a "See also" paragraph. My change makes the [sf.cmath] section less consistent.

Options:

1. Undo the formula reference and add a "See also" paragraph instead.
2. Add a "See also" paragraph and add formula references to all the other functions.
3. Add formula references and remove "See also" paragraphs.

Personally, I find the formula reference to be clearer / more discoverable.

[jensmaurer]

Since we normatively refer to ISO 80000-2, where all these symbols are introduced, I don't think we need to invest extra effort into cross-references. A "see also" seems good enough.

[tkoeppe]

Thanks!