DOC: use variable names in formula for convolve

Use the variables names `a` and `v` as used in the function instead of
using `f` and `g`. This makes it clearer which array "slides" along the other.
Fix typo.
Merge pull request #4047 from arunpersaud/convolve_doc

DOC: use variable names in formula for convolve
Merge pull request #4056 from herr-biber/master

Fix typo.