doc/manual/command-ref/nix-collect-garbage.xml: document --max-freed option
doc/manual/command-ref/nix-collect-garbage.xml: document --max-freed option

Mention the --max-freed option in the nix-collect-garbage man page.
Related to https://github.com/NixOS/nix/issues/609.

ysangkok/nix,layus/nix,dezgeg/nix,vcunat/nix,shlevy/nix,ehmry/nix,shlevy/nix,bennofs/nix,dtzWill/nix,ysangkok/nix,peti/nix,ehmry/nix,zimbatm/nix,bennofs/nix,dtzWill/nix,layus/nix,peti/nix,rycee/nix,ysangkok/nix,vcunat/nix,NixOS/nix,dezgeg/nix,layus/nix,zimbatm/nix,bennofs/nix,dtzWill/nix,vcunat/nix,wmertens/nix,wmertens/nix,dtzWill/nix,rycee/nix,dezgeg/nix,NixOS/nix,rycee/nix,pikajude/nix,wmertens/nix,wmertens/nix,zimbatm/nix,pikajude/nix,layus/nix,pikajude/nix,zimbatm/nix,vcunat/nix,NixOS/nix,shlevy/nix,shlevy/nix,dtzWill/nix,ehmry/nix,peti/nix,rycee/nix,ehmry/nix,rycee/nix,shlevy/nix,ehmry/nix,layus/nix,dezgeg/nix,NixOS/nix,peti/nix,peti/nix,NixOS/nix,bennofs/nix,ehmry/nix,dezgeg/nix