Note SLANG_WARNING_LEVEL_DEFAULT in WarningLevel option docs

Document the fourth SlangWarningLevel enumerator (DEFAULT) as the
always-on group that is a no-op for the WarningLevel option.

Author: expipiplus1

docs/user-guide/08-compiling.md

@@ -1072,7 +1072,7 @@ meanings of their `CompilerOptionValue` encodings.
 | DisableWarnings    | Specifies a list of warnings to disable. `stringValue0` encodes comma separated list of warning codes or names. |
 | EnableWarning      | Specifies a list of warnings to enable. `stringValue0` encodes comma separated list of warning codes or names. |
 | DisableWarning     | Specify a warning to disable. `stringValue0` encodes the warning code or name. |
-| WarningLevel       | Enable a group of opt-in warnings, modeled on clang/gcc. `intValue0` encodes a `SlangWarningLevel` group (`SLANG_WARNING_LEVEL_ALL`/`_EXTRA`/`_PEDANTIC`). Repeatable and additive, matching the `-Wall`/`-Wextra`/`-Wpedantic` command-line flags. The groups are independent (not nested): `extra` is on by default while `pedantic` is off by default, and warnings in the always-on default group are unaffected. |
+| WarningLevel       | Enable a group of opt-in warnings, modeled on clang/gcc. `intValue0` encodes a `SlangWarningLevel` group (`SLANG_WARNING_LEVEL_ALL`/`_EXTRA`/`_PEDANTIC`; `SLANG_WARNING_LEVEL_DEFAULT` is the always-on group and is a no-op here). Repeatable and additive, matching the `-Wall`/`-Wextra`/`-Wpedantic` command-line flags. The groups are independent (not nested): `extra` is on by default while `pedantic` is off by default, and warnings in the always-on default group are unaffected. |
 | ReportDownstreamTime | Turn on/off downstream compilation time report. `intValue0` encodes a bool value for the setting. |
 | ReportPerfBenchmark | Turn on/off reporting of time spent in different parts of the compiler. `intValue0` encodes a bool value for the setting. |
 | SkipSPIRVValidation | Specifies whether or not to skip the validation step after emitting SPIR-V. `intValue0` encodes a bool value for the setting. |