[flang][NFC] Fix fir::ConvertOp description

[peixin]

Summary

According to the support of fir::ConvertOp conversion in CodeGen.cpp,
it also supports integer to pointer conversion and pointer to integer
conversion. The entity can be array, and the conversion type can be
pointer to array.

[jeanPerier]

Thanks

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

def fir_AddrOfOp : fir_OneResultOp<"address_of", [NoSideEffect]> {
   let arguments = (ins SymbolRefAttr:$symbol);
 
   let results = (outs AnyAddressableLike:$resTy);
 
   let assemblyFormat = "`(` $symbol `)` attr-dict `:` type($resTy)";
 }
 
 def fir_ConvertOp : fir_OneResultOp<"convert", [NoSideEffect]> {
-  let summary = "encapsulates all Fortran scalar type conversions";
+  let summary = "encapsulates all Fortran entity type conversions";
 
   let description = [{
     Generalized type conversion. Convert the ssa value from type T to type U.
     Not all pairs of types have conversions. When types T and U are the same
-    type, this instruction is a NOP and may be folded away.
+    type, this instruction is a NOP and may be folded away. This also supports
+    integer to pointer conversion and pointer to integer conversion.
 
     ```mlir
       %v = ... : i64
       %w = fir.convert %v : (i64) -> i32
     ```
 
     The example truncates the value `%v` from an i64 to an i32.
   }];