Merge bitcoin/bitcoin#29486: build: remove -Wdocumentation conditional

b052b2d build: remove -Wdocumentation conditional (fanquake)

Pull request description:

  Now that `--enable-suppress-external-warnings` is on by default, we can drop it. CIs are all already building with this flag.

ACKs for top commit:
  Empact:
    Code review ACK bitcoin/bitcoin@b052b2d
  theuni:
    utACK b052b2d

Tree-SHA512: 8b55f366dfeece082090fb87de67d8811967f4c89987a346431b2deb73c3c94401b59ec98bb1cbf790e18894f3d4c4aebb57cbc5fbf931c1046bf40239bc7a58

Author: fanquake

configure.ac

@@ -432,10 +432,7 @@ if test "$CXXFLAGS_overridden" = "no"; then
   AX_CHECK_COMPILE_FLAG([-Wsuggest-override], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wsuggest-override"], [], [$CXXFLAG_WERROR])
   AX_CHECK_COMPILE_FLAG([-Wimplicit-fallthrough], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wimplicit-fallthrough"], [], [$CXXFLAG_WERROR])
   AX_CHECK_COMPILE_FLAG([-Wunreachable-code], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wunreachable-code"], [], [$CXXFLAG_WERROR])
-
-  if test "$suppress_external_warnings" != "no" ; then
-    AX_CHECK_COMPILE_FLAG([-Wdocumentation], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wdocumentation"], [], [$CXXFLAG_WERROR])
-  fi
+  AX_CHECK_COMPILE_FLAG([-Wdocumentation], [WARN_CXXFLAGS="$WARN_CXXFLAGS -Wdocumentation"], [], [$CXXFLAG_WERROR])
 
   dnl Some compilers (gcc) ignore unknown -Wno-* options, but warn about all
   dnl unknown options if any other warning is produced. Test the -Wfoo case, and