Incorporate macLibREADME text into 19.16

[Andrew Johnson]

There is text in base/src/libCom/macLib/macLibREADME that can probably go straight into the appDevGuide, section 19.16. The wording in macLibNOTES needs a bit of rewriting but what it discusses should also be covered in there somewhere.

[Andrew Johnson (anj)]

Additional text is needed to cover sub-macros $(name=default, sub=value, sub2) and document the library's response to recursive and undefined macros.