Rename glossary files to more meaningful names

The directory and filename were decided based on how the infra
translation script handles it, but the glossary files are not
synced with Zanata automatically and manually maintained.

It looks better to use more meaningful file names.

Even if the glossary files are synced in future, the syncing process
would be different from usual trnaslation sync, so I believe
changing the file path would be no problem.

Change-Id: Ib00e0cab46e2ba624eaf30d2fe3127979090c444

doc/source/glossary.rst

@@ -9,7 +9,7 @@ Glossary of your language
 -------------------------
 
 A glossary file of your language is found at
-``i18n/locale/<lang>/LC_MESSAGES/i18n.po`` in the i18n repository.
+``glossary/locale/<lang>/LC_MESSAGES/glossary.po`` in the i18n repository.
 The file is a usual PO file.
 
 When you want to update a glossary of your language,
@@ -33,7 +33,7 @@ Master glossary
 ---------------
 
 When you add a new entry to the glossary, it is highly recommended to add
-the entry to the master glossary ``i18n/locale/i18n.pot``.
+the entry to the master glossary ``glossary/locale/glossary.pot``.
 By doing this, all language teams can share the glossary.
 
 The master glossary is an usual POT file.