Explicitly document functions generated with macro ART_GET_FIELD_FROM_CODE.

The ART_GET_FIELD_FROM_CODE macro is used to generate the following
set of functions:

  art{Get,Set}<Kind>{Static,Instance}FromCode
  art{Get,Set}<Kind>{Static,Instance}FromCompiledCode

where <Kind> is in {Byte,Boolean,Short,Char,32,64,Obj}.

However, finding the definitions of these functions from their names
was difficult, as these definitions (and their name) are
generated. This change explicitly mentions the name of the functions
generated with macro ART_GET_FIELD_FROM_CODE in a commment, in order
to improve their grep-ability.

Test: mmma art
Change-Id: I22bf4851c562801c491ccdea2d9d9c9f965b9a6f