doc: retroactively document updates to the warning options

Retroactively document the following changes to the warning options:

  - gcc-like syntax (-Wfoo, -Wno-foo)
  - "all" alias
  - "error" metawarning

Added in 2.00rc1 but never documented.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/doc/changes.src b/doc/changes.src
index d32aaf5..02446df 100644
--- a/doc/changes.src
+++ b/doc/changes.src
@@ -288,6 +288,14 @@
 
 \b Significant performance improvements.
 
+\b \c{-w+warning} and \c{-w-warning} can now be written as -Wwarning and
+ -Wno-warning, respectively.  See \k{opt-w}.
+
+\b Add \c{-w+error} to treat warnings as errors.  See \k{opt-w}.
+
+\b Add \c{-w+all} and \c{-w-all} to enable or disable all suppressible
+ warnings.  See \k{opt-w}.
+
 
 \H{cl-0.98.xx} NASM 0.98 Series