[Docs] Add note clarifying PLIST_VERSION field (#10834)

Added a note to clarify that PLIST_VERSION in GoogleService-Info.plist does not refer to the <plist version="1.0"> declaration in a Property List file.

PLIST_VERSION is a Firebase field within the <dict></dict> in the plist file and is accurately listed as being unused.

#no-changelog

Author: andrewheard

docs/FirebaseOptionsPerProduct.md

@@ -51,6 +51,8 @@ Proposal: Deprecate these in the SDK and stop generating them for GoogleService-
 Proposal: Stop generating these for GoogleService-Info.plist.
 
  * PLIST_VERSION
+   * Note: This is different from the `<plist version="1.0">` declaration that is part of the
+     [Property List XML Schema](https://www.apple.com/DTDs/PropertyList-1.0.dtd).
  * IS_ADS_ENABLED
  * IS_ANALYTICS_ENABLED
  * IS_APPINVITE_ENABLED