Add comments explaining regex capture groups in lit.cfg

Author: ramonasuncion

test/lit.cfg

@@ -2938,6 +2938,9 @@ config.substitutions.append(('%target-run-stdlib-swift', config.target_run_stdli
 config.substitutions.append(('%target-repl-run-simple-swift', subst_target_repl_run_simple_swift))
 config.substitutions.append(('%target-run', config.target_run))
 config.substitutions.append(('%target-jit-run', subst_target_jit_run))
+# Capture groups:
+# \1 = path before the file name (non-greedy)
+# \2 = file name (last component, no slashes or parentheses)
 config.substitutions.append(('%target-build-swift-dylib\(([^)]+?)([^/\\()]+)\)', config.target_build_swift_dylib))
 config.substitutions.append(('%target-codesign', config.target_codesign))
 config.substitutions.append(('%target-build-swift', config.target_build_swift))