clarify resource identifier docs

Fixes: #dandi#1435

dandi/cli/cmd_download.py

@@ -18,6 +18,8 @@
     help=f"""\
 Download files or entire folders from DANDI.
 
+\b
+{_dandi_url_parser.resource_identifier_primer}
 \b
 {_dandi_url_parser.known_patterns}
     """

dandi/cli/cmd_ls.py

@@ -26,6 +26,8 @@
 The arguments may be either resource identifiers or paths to local
 files/directories.
 
+\b
+{_dandi_url_parser.resource_identifier_primer}
 \b
 {_dandi_url_parser.known_patterns}
 """

dandi/dandiarchive.py

@@ -675,6 +675,15 @@ class _dandi_url_parser:
             "https://<server>/...",
         ),
     ]
+    resource_identifier_primer = """dandi commands and accept URLs and URL-like identifiers called
+<resource ids> in the following formats for identifying Dandisets, assets, and asset
+collections.
+
+Text in [brackets] is optional.  A server field is a base API or GUI URL
+for a DANDI Archive instance.  If an optional ``version`` field is omitted from
+a URL, the given Dandiset's most recent published version will be used if it
+has one, and its draft version will be used otherwise.
+"""
     known_patterns = "Accepted resource identifier patterns:" + "\n - ".join(
         [""] + [display for _, _, display in known_urls]
     )