Document optional comma in linker script.

The linker script documentation does not mention the optional comma
that may follow an output section command or an overlay command.
In some cases, where a fill expression is used, and the next
output section command begins with an operator (e.g., "/DISCARD/"),
the comma may be required to separate the two commands.

Currently, GNU ld doesn't require the comma, but gold does.

ld/
	PR gold/16804
	* ld.texinfo: Document optional comma following output section
	command and overlay command.