linkcheck: documentation and validation suggestions for linkcheck_allowed_redirects (#13458)

Co-authored-by: Adam Turner <[email protected]>

Author: jayaddison

doc/usage/configuration.rst

@@ -3642,7 +3642,6 @@ and which failures and redirects it ignores.
 
 .. confval:: linkcheck_allowed_redirects
    :type: :code-py:`dict[str, str]`
-   :default: :code-py:`{}`
 
    A dictionary that maps a pattern of the source URI
    to a pattern of the canonical URI.

sphinx/builders/linkcheck.py

@@ -754,7 +754,11 @@ def compile_linkcheck_allowed_redirects(app: Sphinx, config: Config) -> None:
         config.linkcheck_allowed_redirects = None
         return
     if not isinstance(config.linkcheck_allowed_redirects, dict):
-        raise ConfigError
+        msg = __(
+            f'Invalid value `{config.linkcheck_allowed_redirects!r}` in '
+            'linkcheck_allowed_redirects. Expected a dictionary.'
+        )
+        raise ConfigError(msg)
     allowed_redirects = {}
     for url, pattern in config.linkcheck_allowed_redirects.items():
         try: