[flang][docs] Fix cmake warning when building documentation
CMake issues the following warning when configuring with
documentation enabled.
Exactly one of PRE_BUILD, PRE_LINK, or POST_BUILD must be given.
Assuming
POST_BUILD to preserve backward compatibility.
The offending add_custom_command call in flang/docs/CMakeLists.txt has
been fixed to ensure that this warning is not issued. The comment
describing the purpose of the command now provides more details.