Document escaping

Document that strings can contain double quotes escaped as \", and
the droiddoc args argument can contain a dollar signe escaped as $$.

Test: none
Change-Id: I91d3fa73f484094c9ce8cbecc93189850856de84

README.md

@@ -81,6 +81,8 @@ types are:
 Maps may values of any type, including nested maps.  Lists and maps may have
 trailing commas after the last value.
 
+Strings can contain double quotes using `\"`, for example `"cat \"a b\""`.
+
 ### Operators
 
 Strings, lists of strings, and maps can be appended using the `+` operator.

java/droiddoc.go

@@ -211,6 +211,7 @@ type JavadocProperties struct {
 	// Available variables for substitution:
 	//
 	//  $(location <label>): the path to the arg_files with name <label>
+	//  $$: a literal $
 	Args *string
 
 	// names of the output files used in args that will be generated