pw_snapshot: Update software_version comments

Updates the inline documentation for the software_version field of the
snapshot Metadata proto message to clarify that the field is
intentionally not tokenizable.

Change-Id: I26498e88d310c537f087d33a2762e3bffc45bb34
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/100800
Reviewed-by: Carlos Chinchilla <cachinchilla@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Armando Montanez <amontanez@google.com>

pw_snapshot/pw_snapshot_protos/snapshot_metadata.proto

@@ -48,8 +48,11 @@ message Metadata {
  bytes project_name = 3 [(tokenizer.format) = TOKENIZATION_OPTIONAL];
 
  // Version characters must be alphanumeric, punctuation, and space. This
- // string is case-sensitive. This should either be human readable text, or
- // tokenized data.
+ // string is case-sensitive. This should always be human readable text, and
+ // does not support tokenization by design. If this field was tokenized, it's
+ // possible that the token could be lost (e.g. generated by a local developer
+ // build and not uploaded anywhere) and a firmware version running on a device
+ // in the field would be left entirely unidentifiable.
  //
  // Examples:
  // "codename-local-[build_id]"