xdg-shell: relax grabbing popup parent requirements

Prior to b0665014 (xdg-shell: Delete duplicate paragraph in xdg_popup) there were two paragraphs specifying the requirements to the parent of a grabbing popup, which were contradicting each other (see #82). The referenced commit removed the paragraph which effectively stated that a non-grabbing popup can be a grabbing popup's parent if there are no explicit grabs already taken. However, as suggested by #82 (comment 1274459) and weston!1607 (merged), the intention was to specifically allow the mentioned case.

Aside from that, the current description suggests that a parent of a grabbing popup must have the xdg_toplevel or the xdg_popup role. This restriction means that a surface with some other role which can be a parent of an xdg_popup can't be a parent of grabbing popup, which doesn't seem to be intentional.

With this commit, the requirement for a grabbing popup's parent is that it must be a grabbing popup if there is an existing explicit grab. A sentence in the second-to-last paragraph which duplicated the current description has been removed.

Additionally, this commit fixes a typo in the last paragraph (top most -> topmost).