DOC: Better document naming convention

[NickleDave]

Seems like we still need to do a better job of documenting the naming convention that vak expects when it matches annotation files with the files they annotate

Re-reading this "naming convention" callout on the how-to page, it does seem like it could be clearer.
https://vak.readthedocs.io/en/latest/howto/howto_user_annot.html#method-1-converting-your-annotations-to-the-simple-seq-format

And it might help to add some reasoning for the convention?
#523 (comment)

But we could do more.
As suggested by @sthaar:

#523 (comment)

The explanation would help, but you'd still need to look for it. Maybe make it more explicit in the tutorial? And/or refer to it in the error message?

edit: split into a separate issue (#525 ) that has to do with refactoring code

• make "naming convention" section more discoverable -- have it be a section header instead of a call-out so we can easily link directly to it, e.g. in an error message as described in CLN: rename/refactor recursive_stem and add clearer error message #525
• make naming convention explicit in tutorial -- link to the section on how-to page?
• give @sthaar docs credit