doc/lua-filters.md: fix, improve docs for pandoc.mediabag.fetch

doc/lua-filters.md

@@ -3176,12 +3176,22 @@ Usage:
 
 ### fetch {#pandoc.mediabag.fetch}
 
-`fetch (source, base_url)`
+`fetch (source)`
 
 Fetches the given source from a URL or local file. Returns two
 values: the contents of the file and the MIME type (or an empty
 string).
 
+The function will first try to retrieve `source` from the
+mediabag; if that fails, it will try to download it or read it
+from the local file system while respecting pandoc's "resource
+path" setting.
+
+Parameters:
+
+`source`:
+:   path to a resource; either a local file path or URI
+
 Returns:
 
 -   the entries MIME type, or nil if the file was not found.
@@ -3190,7 +3200,7 @@ Returns:
 Usage:
 
     local diagram_url = "https://pandoc.org/diagram.jpg"
-    local mt, contents = pandoc.mediabag.fetch(diagram_url, ".")
+    local mt, contents = pandoc.mediabag.fetch(diagram_url)
 
 # Module pandoc.List