Document --independent-groups

...in the hspace man page. Also document the default behavior
of hspace if this option is not given.
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Hrvoje Ribicic <riba@google.com>

man/hspace.rst

@@ -25,6 +25,7 @@ Algorithm options:
 **[ \--max-cpu *cpu-ratio* ]**
 **[ \--min-disk *disk-ratio* ]**
 **[ -O *name...* ]**
+**[ \--independent-groups ]**
 
 
 Request options:
@@ -232,6 +233,12 @@ The options that can be passed to the program are as follows:
   number. For example, specifying *disk-ratio* as **0.25** means that
   at least one quarter of disk space should be left free on nodes.
 
+\--independent-groups
+  Consider all groups independent. That is, if a node that is not N+1
+  happy is found, ignore its group, but still do allocation in the other
+  groups. The default is to not try allocation at all, if some not N+1
+  happy node is found.
+
 -l *rounds*, \--max-length=*rounds*
   Restrict the number of instance allocations to this length. This is
   not very useful in practice, but can be used for testing hspace