Gem.use_gemdeps still uses RUBYGEMS_GEMDEPS

So document that
Raise ArgumentError when gem deps file is missing

Now that Gem.use_gemdeps prefers an argument it should raise an
exception if the file given should not be found.

Since the method falls back to the RUBYGEMS_GEMDEPS environment variable
(which is used via gem executable stubs) the method does not raise an
exception when the argument matches the RUBYGEMS_GEMDEPS environment
variable.

This is not great because sometimes an exception will not be raised
which will be confusing, but it does maintain backwards compatibility.
Consistent exceptions from Gem.use_gemdeps

Now an exception is always raised when an argument is given and not
found.

To maintain compatibility an exception is not raised when the method
falls back to the RUBYGEMS_GEMDEPS environment variable.