git-fast-export.txt: clarify why 'verbatim' may not be a good idea

Signed-off-by: Luke Shumaker <[email protected]>
Signed-off-by: Christian Couder <[email protected]>

Author: LukeShu

Documentation/git-fast-export.adoc

@@ -29,15 +29,19 @@ OPTIONS
 
 --signed-tags=(verbatim|warn-verbatim|warn-strip|strip|abort)::
 	Specify how to handle signed tags.  Since any transformation
-	after the export can change the tag names (which can also happen
-	when excluding revisions) the signatures will not match.
+	after the export (or during the export, such as excluding
+	revisions) can change the hashes being signed, the signatures
+	may become invalid.
 +
 When asking to 'abort' (which is the default), this program will die
 when encountering a signed tag.  With 'strip', the tags will silently
 be made unsigned, with 'warn-strip' they will be made unsigned but a
 warning will be displayed, with 'verbatim', they will be silently
 exported and with 'warn-verbatim' (or 'warn', a deprecated synonym),
-they will be exported, but you will see a warning.
+they will be exported, but you will see a warning.  'verbatim' and
+'warn-verbatim' should only be used if you know that no
+transformation affecting tags will be performed, or if you do not
+care that the resulting tag will have an invalid signature.
 
 --tag-of-filtered-object=(abort|drop|rewrite)::
 	Specify how to handle tags whose tagged object is filtered out.