Document --max-freed for nix-collect-garbage (#12155)

* Update nix-collect-garbage.md

Referencing issue at: https://github.com/NixOS/nix/issues/12132

Copied the description of `--max-freed` option from 442a2623e4/doc/manual/source/command-ref/nix-store/gc.md (L39-L44)

doc/manual/source/command-ref/nix-collect-garbage.md

@@ -62,6 +62,15 @@ These options are for deleting old [profiles] prior to deleting unreachable [sto
   This is the equivalent of invoking [`nix-env --delete-generations <period>`](@docroot@/command-ref/nix-env/delete-generations.md#generations-time) on each found profile.
   See the documentation of that command for additional information about the *period* argument.
 
+  - <span id="opt-max-freed">[`--max-freed`](#opt-max-freed)</span> *bytes*
+
+<!-- duplication from https://github.com/NixOS/nix/blob/442a2623e48357ff72c77bb11cf2cf06d94d2f90/doc/manual/source/command-ref/nix-store/gc.md?plain=1#L39-L44 -->
+
+  Keep deleting paths until at least *bytes* bytes have been deleted,
+  then stop. The argument *bytes* can be followed by the
+  multiplicative suffix `K`, `M`, `G` or `T`, denoting KiB, MiB, GiB
+  or TiB units.
+  
 {{#include ./opt-common.md}}
 
 {{#include ./env-common.md}}