Add documentation for module resolution.

# The base list of modules to build for this product is specified

# by the appropriate product definition file, which was included

# by product_config.mk.

# Name resolution for PRODUCT_PACKAGES:

#   foo:32 resolves to foo_32;

#   foo:64 resolves to foo;

#   foo resolves to both foo and foo_32 (if foo_32 is defined).

#

# Name resolution for LOCAL_REQUIRED_MODULES:

#   If a module is built for 2nd arch, its required module resolves to

#   32-bit variant, if it exits. See the select-bitness-of-required-modules definition.

# $(1): product makefile

define product-installed-files

  $(eval _pif_modules := $(PRODUCTS.$(strip $(1)).PRODUCT_PACKAGES)) \