[fir][NFC] Update doc for pinned attr in fir.alloca

[clementval]

Summary

Add descritpion for the attribute added in D110815.

[kiranchandramohan]

Thanks @clementval for adding the doc and also for upstreaming the pinned attribute change. Have Nit comment.

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

128

[clementval]

Address nit comment

[clementval]

In D110877#3034780, @kiranchandramohan wrote:

Thanks @clementval for adding the doc and also for upstreaming the pinned attribute change. Have Nit comment.

Thanks for the reviews. By the way, I'm trying to preserve as much as possible the authorship from fir-dev. So the pinned attr was committed with you as the author.

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

let description = [{
     implementation could therefore convert stack allocations to global
     allocations. Such a conversion effectively adds the SAVE attribute to all
     variables.
 
     Some temporary entities (large arrays) probably should not be stack
     allocated as stack space can often be limited. A legal implementation can
     convert these large stack allocations to heap allocations regardless of
     whether the procedure is recursive or not.
+
+    The pinned attribute is used to flag fir.alloca operation in a specific 
+    region and avoid them to be hoisted in a alloca hoisting pass. 
   }];
 
   let arguments = (ins
     TypeAttr:$in_type,
     OptionalAttr<StrAttr>:$uniq_name,
     OptionalAttr<StrAttr>:$bindc_name,
     UnitAttr:$pinned,
     Variadic<AnyIntegerType>:$typeparams,