[flang][NFC] Add missing sourceBox documentation in embox ops (#101221)

Just noticed that `sourceDoc` was not documented in embox operations
description. this patch just add the documentation.

flang/include/flang/Optimizer/CodeGen/CGOps.td

@@ -48,6 +48,8 @@ def fircg_XEmboxOp : fircg_Op<"ext_embox", [AttrSizedOperandSegments]> {
        - substring: A substring operator (offset, length) for CHARACTER.
        - LEN type parameters: A vector of runtime LEN type parameters that
          describe an correspond to the elemental derived type.
+       - sourceBox: A box to read information from such as CFI type,
+         type descriptor or element size to populate the new descriptor.
        - allocator_idx: specify special allocator to use.
 
     The memref and shape arguments are mandatory. The rest are optional.

flang/include/flang/Optimizer/Dialect/FIROps.td

@@ -782,6 +782,8 @@ def fir_EmboxOp : fir_Op<"embox", [NoMemoryEffect, AttrSizedOperandSegments]> {
           lower bounds and extents may not be known until runtime),
         - slice: an array section can be described with a slice triple,
         - typeparams: for emboxing a derived type with LEN type parameters,
+        - sourceBox: A box to read information from such as CFI type,
+          type descriptor or element size to populate the new descriptor.
         - accessMap: unused/experimental.
         - allocator_idx: specify special allocator to use.
   }];