Invalid comment cleanup (#4836)

This is a followup from #4834, I searched for "invalid" uses in our
codebase. This is mostly changing comments, and a couple debug
functions, but shouldn't affect testable behavior.

Note a couple things I'll highlight as not changing (but could) are:
- `ReturnTypeInfo::is_valid`
- `"invalid"` uses in the formatter
- `AddInvalid` for `!has_value` in `inst_fingerprinter` (because the
cases it's called sound invalid-ish)

toolchain/check/pattern_match.cpp

@@ -56,12 +56,12 @@ class MatchContext {
  public:
   struct WorkItem {
     SemIR::InstId pattern_id;
-    // Invalid when processing the callee side.
+    // `None` when processing the callee side.
     SemIR::InstId scrutinee_id;
   };
 
-  // Constructs a MatchContext. If `callee_specific_id` is valid, this pattern
-  // match operation is part of implementing the signature of the given
+  // Constructs a MatchContext. If `callee_specific_id` is not `None`, this
+  // pattern match operation is part of implementing the signature of the given
   // specific.
   explicit MatchContext(MatchKind kind, SemIR::SpecificId callee_specific_id =
                                             SemIR::SpecificId::None)