import-hg: Update example to work with a current hg-fast-export

Version 171002 of hg-fast-export changed the format of mapping files
to allow arbitrary strings as input and output. Therefore update the
example and the description of the format to avoid user
confusion (frej/fast-export#116).

Author: frej

book/09-git-and-other-scms/sections/import-hg.asc

@@ -38,16 +38,18 @@ Joe Smith <[email protected]>
 ----
 
 In this example, the same person (Bob) has created changesets under four different names, one of which actually looks correct, and one of which would be completely invalid for a Git commit.
-Hg-fast-export lets us fix this by adding `={new name and email address}` at the end of every line we want to change, and removing the lines for any usernames that we want to leave alone.
+Hg-fast-export lets us fix this by turning each line into a rule: `"<input>"="<output>"`, mapping an `<input>` to an `<output>`.
+Inside the `<input>` and `<output>` strings, all escape sequences understood by the python `string_escape` encoding are supported.
+If the author mapping file does not contain a matching `<input>`, that author will be sent on to Git unmodified.
 If all the usernames look fine, we won't need this file at all.
 In this example, we want our file to look like this:
 
 [source]
 ----
-bob=Bob Jones <[email protected]>
-bob@localhost=Bob Jones <[email protected]>
-bob <[email protected]>=Bob Jones <[email protected]>
-bob jones <bob <AT> company <DOT> com>=Bob Jones <[email protected]>
+"bob"="Bob Jones <[email protected]>"
+"bob@localhost"="Bob Jones <[email protected]>"
+"bob <[email protected]>"="Bob Jones <[email protected]>"
+"bob jones <bob <AT> company <DOT> com>"="Bob Jones <[email protected]>"
 ----
 
 The next step is to create our new Git repository, and run the export script: