Clarify the usage of "limit" and "fixed" in GIT ranges.

[calebbrown]

The range events are not clear on how "limit" and "fixed" should be used in relation to each other (if they even should).

Lets say I have the following tree of commits:

A <- B <- C <- D
     + <- E
     + <- F <- G

I also have an OSV range with the following events for the above repo

[
  { "introduced": "A" },
  { "limit": "D" },
  { "fixed": "G" }
]

The schema is not clear on the correct way to interpret this range, or whether or not this is even acceptable. For example:

• Is the commit "F" considered vulnerable?
• What about "E"?

The enumeration code in https://github.com/google/osv.dev treats them mutually exclusively, with "limit" taking preference.

Please update the schema to make this usage clearer.

#169 may be related.