Rollup merge of #115487 - ModProg:patch-1, r=dtolnay

Improve documentation on when signes are printed by default

I found the original formulation a bit irritating, but not sure if I really improved it.

Author: matthiaskrgr

library/alloc/src/fmt.rs

@@ -177,8 +177,8 @@
 //! These are all flags altering the behavior of the formatter.
 //!
 //! * `+` - This is intended for numeric types and indicates that the sign
-//!         should always be printed. Positive signs are never printed by
-//!         default, and the negative sign is only printed by default for signed values.
+//!         should always be printed. By default only the negative sign of signed values
+//!         is printed, and the sign of positive or unsigned values is omitted.
 //!         This flag indicates that the correct sign (`+` or `-`) should always be printed.
 //! * `-` - Currently not used
 //! * `#` - This flag indicates that the "alternate" form of printing should