Link and exec documentation updated.

Clarifies the usage of exec and link that was confusing me.
Johan Andersson <notifications@github.com> writes:

>> @@ -310,7 +311,9 @@ cask link
>>     cask [GLOBAL-OPTIONS] link list
>>     cask [GLOBAL-OPTIONS] link delete PACKAGE
>>  
>> -Handle package links.
>> +Link between this package and a dependency on the local filesystem. A linked
>> +dependency avoids the need to download a dependency from a remote archive. The
>> +package linked to must also be managed by cask.
>
> That last sentence is actually not true. The requirement is that the package
> has a `Cask`-file or `-pkg.el` file.

Yep, I discovered this since this morning. I was getting a crash from
cask while trying to link to a package of mine. I think this was a
problem with my -pkg.el file, though, as linking a clean directory cask
gives a nice error message describing what you have said.

Phil