Apply suggestions from code review

Co-authored-by: Šarūnas Nejus <snejus@protonmail.com>

Author: ShimmerGlass

beets/util/config.py

@@ -29,15 +29,14 @@ def sanitize_choices(
 def sanitize_pairs(
     pairs: Sequence[tuple[str, str]],
     pairs_all: Sequence[tuple[str, str]],
-    raise_on_unknown=False,
+    raise_on_unknown: bool=False,
 ) -> list[tuple[str, str]]:
     """Clean up a single-element mapping configuration attribute as returned
     by Confuse's `Pairs` template: keep only two-element tuples present in
     pairs_all, remove duplicate elements, expand ('str', '*') and ('*', '*')
     wildcards while keeping the original order. Note that ('*', '*') and
     ('*', 'whatever') have the same effect.
-    Giving raise_on_unknown=True raises an UnknownPairError when a value in
-    pairs does not exist in pairs_all.
+    Set raise_on_unknown to raise an error when a provided pair is not recognised
 
     For example,