Rollup merge of #132851 - chansuke:update-comment, r=thomcc

Update the doc comment of `ASCII_CASE_MASK`

Revived and continued the work from #120282.

the original [branch](https://github.com/mahmudsudo/rust-1/tree/patch-1) was deleted, i created a new branch to carry the changes forward

library/core/src/num/mod.rs

@@ -528,7 +528,7 @@ impl isize {
     midpoint_impl! { isize, signed }
 }
 
-/// If the 6th bit is set ascii is lower case.
+/// If the bit selected by this mask is set, ascii is lower case.
 const ASCII_CASE_MASK: u8 = 0b0010_0000;
 
 impl u8 {