This is mostly stylistic but also adds a couple of missing comments.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* libgnat/s-valuer.adb (Scan_Decimal_Digits): Consistently avoid
	initializing local variables.
	(Scan_Integral_Digits): Likewise.
	(Scan_Raw_Real): Likewise and add a couple of comments.