[analyzer] Extend the Representing Values section of the dev manual.
[analyzer] Plist: change the type of issue_hash from int to string.

This gives more flexibility to what could be stored as issue_hash.
[analyzer] Include the bug uniqueing location in the issue_hash.

The issue here is that if we have 2 leaks reported at the same line for
which we cannot print the corresponding region info, they will get
treated as the same by issue_hash+description. We need to AUGMENT the
issue_hash with the allocation info to differentiate the two issues.

Add the "hash" (offset from the beginning of a function) representing
allocation site to solve the issue.

We might want to generalize solution in the future when we decide to
track more than just the 2 locations from the diagnostics.