If a term can mean different things, then they need to be disambiguated. For a thorough discussion of disambiguation, see Wikipedia:WP:DAB, because MathHelp follows many of these guidelines. Our policy is simpler: don't link to a disambiguation page unless you really intend to do so. Signal your intention to link to a disambiguation page by linking as [[fubar (disambiguation)|]], creating a redirect as needed.
Here's the rationale for this policy:
Suppose a page on "Fubar's theorem" is written, and becomes part of the wiki. Maybe a bunch of other pages are then written, and link to Fubar's theorem. Finally, it is noted by an editor that Fubar has two theorems, so he "moves" the Fubar's theorem page to Fubar's first theorem, leaving a redirect behind so that all the pages linking to Fubar's theorem don't get lost.
Do you follow me so far?
OK, now, this editor writes a new article called Fubar's second theorem, and then he sees the need for a disambiguation page, so he deletes the redirect called "Fubar's theorem", and replaces it with a disambiguation page that briefly describes the two theorems, and links to them both. Is he done? No. He needs to click "what links here" to find links to his disambiguation page, and point them all to one theorem or the other as needed. But he gets called away, and never finishes the job.
Now it's time to write an article that intentionally links to the disambiguation page. It says something like "there are [[Fubar's theorem|two theorems named after Fubar]]". Now there are lots of pages that link to this disambiguation page, one of which is intentional. Wouldn't it be nice to somehow "mark" the intentional link as such? Here's how: First, create a new redirect called "Fubar's theorem (disambiguation)" that points to the disambiguation page, Fubar's theorem. Then, in the article that intentionally links to the disambiguation page, write "there are [[Fubar's theorem (disambiguation)|two theorems named after Fubar]]".