(GDB/MI Compatibility with CLI): Qualify more carefully.

(GDB/MI Compatibility with CLI): Qualify more carefully.
(GDB/MI Simple Examples): Use @subheading for "A Bad Command".
(GDB/MI Data Manipulation): Remove description of unimplemented
display related commands as variable objects perform this function
and are superior: -display-delete, -display-disable,
-display-enable, -display-insert and -display-list.  Move
-environment-cd, -environment-directory, -environment-path
and -environment-pwd to "Program Context".
(GDB/MI Program Control): Rename to "Program Execution".  Move
-exec-arguments -exec-abort to...
(GDB/MI Program Context): ...here.  New node split from "Data
 Manipulation".