[mlir]: Clarify docs for external OpTrait::FunctionLike ops

The documentation claims that an op with the trait FunctionLike has a
single region containing the blocks that corresponding to the body of
the function. It then goes on to say that the absence of a region
corresponds to an external function when, in fact, this is represented
by a single empty region. This patch changes the wording in the
documentation to match the implementation.

Signed-off-by: Frej Drejhammar <frej.drejhammar@gmail.com>
Co-authored-by: Frej Drejhammar <frej.drejhammar@gmail.com>
Co-authored-by: Klas Segeljakt <klasseg@kth.se>

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D89868

mlir/docs/Traits.md

@@ -249,7 +249,7 @@ particular:
 -   Ops must be symbols, i.e. also have the `Symbol` trait;
 -   Ops have a single region with multiple blocks that corresponds to the body
     of the function;
--   the absence of a region corresponds to an external function;
+-   An op with a single empty region corresponds to an external function;
 -   arguments of the first block of the region are treated as function
     arguments;
 -   they can have argument and result attributes that are stored in dictionary

mlir/include/mlir/IR/FunctionSupport.h

@@ -79,7 +79,7 @@ namespace OpTrait {
 /// - Ops must be symbols, i.e. also have the `Symbol` trait;
 /// - Ops have a single region with multiple blocks that corresponds to the body
 ///   of the function;
-/// - the absence of a region corresponds to an external function;
+/// - An op with a single empty region corresponds to an external function;
 /// - leading arguments of the first block of the region are treated as function
 ///   arguments;
 /// - they can have argument attributes that are stored in a dictionary