This is actually not a typo :)  The meaning is:

"Tells yum to run entirely from system cache - does not download or update any headers unless it has to [download or update any headers] to perform the requested action."

I can remember myself also pausing for a second when reading this before it finally "clicked", though :)

Would adding a comma after the first "to" make it more readable and/or correct?

Ok, I see now.

Lets try:

1]
Tells yum to run entirely from system cache, does not download or update any headers unless it has to, to perform the requested action.

{I hate the use of dashes and slashes in running text, so that was my first thought}.

2]
Tells yum to run entirely from system cache - does not download or update any headers to perform the requested action unless it has to.

{short is good}

3]
Tells yum to run entirely from system cache; does not download or update any headers to perform the requested action unless it has to.

{but then, ideally, you should change all use of - as sentence break.}

Thank you

(In reply to Stephen Wadeley from comment #3)
> 1]
> Tells yum to run entirely from system cache, does not download or update any
> headers unless it has to, to perform the requested action.

This slightly changes the meaning.  Running from system cache means not downloading any headers (the comma would imply the latter is done in addition to the former).

> {I hate the use of dashes and slashes in running text, so that was my first
> thought}.

I do agree; that eliminates 2] as well.
 
> 3]
> Tells yum to run entirely from system cache; does not download or update any
> headers to perform the requested action unless it has to.

I like this the most :)

I'll make this into a quick patch when time allows...  Thanks!