Update m help text with guidance on parallelism

Remove "-j" from the default recommended arguments and add a paragraph
describing the -j behavior.

Test: m help
Change-Id: Id5a5f4719f18ea2a9d09df43f5fb38592a59f5c9

help.sh

@@ -12,11 +12,15 @@ cd '"${TOP}"'
 source build/envsetup.sh    # Add "lunch" (and other utilities and variables)
                             # to the shell environment.
 lunch [<product>-<variant>] # Choose the device to target.
-m -j [<goals>]              # Execute the configured build.
+m [<goals>]                 # Execute the configured build.
 
 Usage of "m" imitates usage of the program "make".
 See '"${SCRIPT_DIR}"'/Usage.txt for more info about build usage and concepts.
 
+The parallelism of the build can be set with a -jN argument to "m".  If you
+don't provide a -j argument, the build system automatically selects a parallel
+task count that it thinks is optimal for your system.
+
 Common goals are:
 
     clean                   (aka clobber) equivalent to rm -rf out/