composition: Should add explicit (and simplified) behavior of //include/static

[EricCousineau-TRI]

Per f2f today:

• (IMO) Current behavior may be too complex
• //include/static=true should override children
• //include/static=flase will not override children

@azeey's asked if a static model could be a //joint/child.
IMO, yes, because it's analogous to other compatible specifications (e.g. a rope connected at two ends to the world).

Should also re-mention that //joint/child != "world" primarily because of breaking pose encapsulation as our main motivation.

Part of gazebosim/sdformat#278 (but not explicitly included, 'cause it seems simple?)