Add documentation about the purpose of #apex and #symbols in map.txt

Currently we document the semantics of these two symbols. I would like
to add some documentation about why this granularity is necessary. The
majority of the details are contained in b/191371676

Test: N/A (documentation change)
Bug: 265485106
Change-Id: I752c921eabceff94f92f24a46c9a16dc28a2af19