CMake: remove -Dstd-docs=false flag when building stage3

This now defaults to false already since the autodocs rework.

The langref still cannot be enabled by default because the langref
contains doctests that exercise the `@cImport` feature which is disabled
in zig2 builds.

Author: andrewrk

CMakeLists.txt

@@ -923,8 +923,11 @@ if(MSVC OR MINGW)
 endif()
 
 
-# "-Dno-langref" and "-Dstd-docs=false" are hardcoded because they take too long to build.
-# To obtain these two forms of documentation, run zig build against stage3 rather than stage2.
+# "-Dno-langref" is hardcoded because stage2 builds lack the `@cImport`
+# feature, which some of the doctests rely on.
+
+# To obtain this document, run `zig build` against stage3 rather than stage2.
+# Note that the `langref` step can be used to isolate this task.
 set(ZIG_BUILD_ARGS
   --zig-lib-dir "${PROJECT_SOURCE_DIR}/lib"
 
@@ -936,7 +939,6 @@ set(ZIG_BUILD_ARGS
   "-Dconfig_h=${ZIG_CONFIG_H_OUT}"
 
   -Dno-langref
-  -Dstd-docs=false
 )
 
 if("${CMAKE_BUILD_TYPE}" STREQUAL "Debug")