[flang][docs] Do not recommend FLANG_ENABLE_WERROR=ON... (#171152)

...unless the reader is going to contribute to Flang.

Fixes #168911.

In which someone followed the getting started instructions but because
they were using GCC and it produced some warnings, those got upgraded to
errors and failed the build.

The GCC builds of anything in LLVM are always going to have some warning
of some kind, so while it would be nice to fix or silence them, let's
not inflict that on new developers.

This option defaults to `OFF` and is turned `ON` by a handful of bots,
so if the intent was to make developers more attentive to warnings, that
will still happen. Just a bit later, which that's better than putting
them off up front.

If they go on to contribute, they can switch to a Clang build and enable
the option to save themselves some post-commit hassle.

Author: DavidSpickett

flang/docs/GettingStarted.md

@@ -76,7 +76,6 @@ cmake \
   -DCMAKE_INSTALL_PREFIX=$INSTALLDIR \
   -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
   -DCMAKE_CXX_LINK_FLAGS="-Wl,-rpath,$LD_LIBRARY_PATH" \
-  -DFLANG_ENABLE_WERROR=ON \
   -DLLVM_ENABLE_ASSERTIONS=ON \
   -DLLVM_TARGETS_TO_BUILD=host \
   -DLLVM_LIT_ARGS=-v \
@@ -87,6 +86,15 @@ cmake \
 ninja
 ```
 
+```{note}
+  Contributions to Flang are expected not to produce any new compiler warnings.
+  This is enforced by post-commit buildbots. To do the same locally, add
+  `-DFLANG_ENABLE_WERROR=ON` to the above `cmake` command.
+
+  Only Clang builds are checked for this, so we do not recommend using this
+  option with GCC as there will be preexisting warnings.
+```
+
 On Darwin, to make flang able to link binaries with the default sysroot without
 having to specify additional flags, use the `DEFAULT_SYSROOT` CMake flag, e.g.
 `-DDEFAULT_SYSROOT="$(xcrun --show-sdk-path)"`.
@@ -142,7 +150,6 @@ cmake \
   -DCMAKE_BUILD_TYPE=Release \
   -DCMAKE_CXX_LINK_FLAGS="-Wl,-rpath,$LD_LIBRARY_PATH" \
   -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
-  -DFLANG_ENABLE_WERROR=ON \
   -DLLVM_TARGETS_TO_BUILD=host \
   -DLLVM_ENABLE_ASSERTIONS=ON \
   -DLLVM_BUILD_MAIN_SRC_DIR=$ROOTDIR/build/lib/cmake/llvm \
@@ -156,6 +163,15 @@ cmake \
 ninja
 ```
 
+```{note}
+  Contributions to Flang are expected not to produce any new compiler warnings.
+  This is enforced by post-commit buildbots. To do the same locally, add
+  `-DFLANG_ENABLE_WERROR=ON` to the above `cmake` command.
+
+  Only Clang builds are checked for this, so we do not recommend using this
+  option with GCC as there will be preexisting warnings.
+```
+
 To run the flang tests on this build, execute the command in the `flang/build`
 directory:
 ```bash