docs: clarify why negated generates globs are skipped in existence check

Assisted by AI

Co-Authored-By: Claude <noreply@anthropic.com>

Author: drichardson

internal/fingerprint/sources_checksum.go

@@ -53,6 +53,7 @@ func (checker *ChecksumChecker) IsUpToDate(t *ast.Task) (bool, error) {
 	if len(t.Generates) > 0 {
 		// For each specified 'generates' field, check whether the files actually exist
 		for _, g := range t.Generates {
+			// Exclusion patterns don't represent output files; skip them.
 			if g.Negate {
 				continue
 			}

internal/fingerprint/sources_timestamp.go

@@ -37,6 +37,7 @@ func (checker *TimestampChecker) IsUpToDate(t *ast.Task) (bool, error) {
 	// file means the task must run regardless of timestamps.
 	if len(t.Generates) > 0 {
 		for _, g := range t.Generates {
+			// Exclusion patterns don't represent output files; skip them.
 			if g.Negate {
 				continue
 			}