Fix and improve documentation for cover images from buchhandel.de (vufind-org#4587)

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Co-authored-by: Stefan Weil <stefan.weil@uni-mannheim.de>

Author: stweil

config/vufind/config.ini

@@ -1061,9 +1061,10 @@ verify_server_certificate = false
 ;coversize = false
 
 ; You can select Syndetics, LibraryThing, Summon, Booksite, OpenLibrary,
-; Contentcafe, Buchhandel.de, Google, BrowZine, ObalkyKnih, Orb, Koha, Demo,
+; Contentcafe, Buchhandel, Google, BrowZine, ObalkyKnih, Orb, Koha, Demo,
 ; and/or LocalFile. Service-specific notes:
 ;   - BrowZine requires you to have BrowZine.ini configured appropriately.
+;   - Buchhandel requires a valid token in the [Buchhandel] section.
 ;   - Google Books caching behavior can be customized in the [Cache_GoogleCover]
 ;       section.
 ;   - Koha requires the koha_cover_url setting below. Additionally, for
@@ -1100,7 +1101,7 @@ verify_server_certificate = false
 ;   - Orb requires that you complete the [Orb] section. Cache settings can be
 ;       adjusted in the [Cache_OrbCover] section.
 ;   - Summon service takes a Serials Solutions client key, NOT Summon API key!
-;coverimages     = Syndetics:MySyndeticsId,Booksite,LibraryThing:MyLibraryThingId,Google,ObalkyKnih,OpenLibrary,Summon:MySerialsSolutionsClientKey,Contentcafe:MyContentCafeID,BrowZine,LocalFile:PathToFile,Koha,Orb
+;coverimages     = Syndetics:MySyndeticsId,Booksite,LibraryThing:MyLibraryThingId,Google,ObalkyKnih,OpenLibrary,Summon:MySerialsSolutionsClientKey,Buchhandel,Contentcafe:MyContentCafeID,BrowZine,LocalFile:PathToFile,Koha,Orb
 
 ; When using the Koha cover provider, you should fill in this setting:
 ;koha_cover_url = "https://localhost/cgi-bin/koha/opac-image.pl"