KB: attempt updating the OpenMP documentation

Attempt to make the OpenMP entry in the Knowledge Base clearer.
Notably:

- avoid suggesting that OpenMP gets enabled automatically (apparently
  this was the case up to gfortran-9, but not anymore) -- now it needs
  to be enabled by packages explicitly

- indicate which implementations are used by default but make it clear
  that different packages may be forcing other implementations

- explicitly recommend building against libgomp on Linux

- fill Windows support in

Signed-off-by: Michał Górny <mgorny@quansight.com>

Author: mgorny

docs/maintainer/knowledge_base.md

@@ -1162,9 +1162,16 @@ corresponding compilers in `requirements/build` as normal.
 
 ### OpenMP
 
-You can enable OpenMP on macOS by adding the `llvm-openmp` package to the `build` section of the `meta.yaml`.
-For Linux OpenMP support is on by default, however it's better to explicitly depend on the libgomp package which is the OpenMP
-implementation from the GNU project.
+The default OpenMP runtime used depends on the compiler and platform used. GCC (on Linux and the Windows MinGW
+toolchain) uses the `libgomp` runtime. Clang (on all platforms) and GNU Fortran on macOS default to the `llvm-openmp`
+runtime. MSVC may use its own runtime or `llvm-openmp`. However, individual projects may force building against
+a different runtime.
+
+On Linux, building against `libgomp` is recommended as the other OpenMP providers are backwards compatible with it.
+When packages are built against `libgomp`, it is possible to use both `libgomp` and `llvm-openmp` provider at runtime.
+
+Dependencies on OpenMP providers belong in the `build` section of recipe dependencies. The following snippet corresponds
+to the defaults, though individual packages may require a different set:
 
 ```yaml
 # meta.yaml
@@ -1181,7 +1188,10 @@ requirements:
 On macOS, only LLVM's OpenMP implementation `llvm-openmp` is supported. This implementation is used even in Fortran code compiled
 using GNU's gfortran.
 
-On Linux (except aarch64), packages are linked against GNU's `libgomp.so.1`, but the OpenMP library at install time can be
+On Windows, switching between different OpenMP implemnentations is not supported. Every package uses the implementation
+it was built against.
+
+On Linux, when packages are linked against GNU's `libgomp.so.1`, but the OpenMP library at install time can be
 switched from GNU to LLVM by doing the following.
 
 ```shell-session