Doc for extrude() function has confusing description for bidirectionalLength parameter

The current description of bidirectionalLength is:

If specified, will also extrude in the opposite direction to 'distance' to the specified distance. If 'symmetric' is true, this value is ignored.

distance is not a parameter, and it's not 100% clear what it refers to, and it's not clear what "specified distance" refers to.  Moreover the wording is just a bit strange.

