doc: git-add: explain inconsistent terminology

I think the fact that git uses these three terms interchangeably is
extremely confusing and that it deserves to be noted.

Signed-off-by: Julia Evans <[email protected]>

Author: jvns

Documentation/git-add.adoc

@@ -45,6 +45,10 @@ exact filename of an ignored file (e.g. `git add ignored.txt`), `git
 add` will fail with a list of ignored files. Otherwise it will silently
 ignore the file.
 
+[NOTE]
+Git uses the terms "staging area", "index" and "cache" interchangeably
+for historical reasons.
+
 OPTIONS
 -------
 `<pathspec>...`::