Don't include descriptions in the generated DocBlocks

These mostly just duplicate the PHP documenation.  Developers are unlikely to
refer to these for the PHP documentation, and they can still inspect the
return type to confirm that, e.g., `false` will not be returned.

Instead, only include the type information.

This reduces the size of the generated code from 6.27Mb to 2.79Mb.

Part of #706.

Author: JakeQZ