[docs] Add support for Markdown documentation when creating man pages
[docs] Add support for Markdown documentation when creating man pages

rL358749 added a documentation page in the Markdown format. Currently,
such pages are ignored in the configuration script for manual pages.
This patch fixes that.

Differential Revision: https://reviews.llvm.org/D60964


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@359860 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm