Due to changes in gh-19118, classes that contain @Lookup methods are
no longer required to be concrete classes for use with component
scanning; however, the reference documentation still states that such
classes must not be abstract.
This commit therefore removes the outdated reference documentation and
updates the corresponding Javadoc.
See gh-19118
Closes gh-34367
(cherry picked from commit 819a7c86c1)
b7a996a64b