[docs] LibraryEvolution: Merge @alwaysEmitIntoClient into @inlineable. (#6118)

JoeG (and Dmitri before him) pointed out a while back that having
/two/ ways to get something emitted into a client's module was really
more complexity than we really needed, and created a pitfall for
library authors who might choose the wrong one for their needs.

Author: jrose-apple