Improve the documentation of the set add/remove methods.

Signed-off-by: Ewan Mellor <ewan@xensource.com>

docs/xen-api/xenapi-datamodel.tex

@@ -2635,7 +2635,7 @@ void
 \subsubsection{RPC name:~add\_VCPUs\_features\_force\_on}
 
 {\bf Overview:} 
-set add message derived from field VCPUs/features/force\_on of object VM
+Add the given value to the VCPUs/features/force\_on field of the given VM.  If the value is already in that Set, then do nothing.
 
  \noindent {\bf Signature:} 
 \begin{verbatim} void add_VCPUs_features_force_on (session_id s, VM ref self, cpu_feature value)\end{verbatim}
@@ -2669,7 +2669,7 @@ void
 \subsubsection{RPC name:~remove\_VCPUs\_features\_force\_on}
 
 {\bf Overview:} 
-set remove message derived from field VCPUs/features/force\_on of object VM
+Remove the given value from the VCPUs/features/force\_on field of the given VM.  If the value is not in that Set, then do nothing.
 
  \noindent {\bf Signature:} 
 \begin{verbatim} void remove_VCPUs_features_force_on (session_id s, VM ref self, cpu_feature value)\end{verbatim}
@@ -2769,7 +2769,7 @@ void
 \subsubsection{RPC name:~add\_VCPUs\_features\_force\_off}
 
 {\bf Overview:} 
-set add message derived from field VCPUs/features/force\_off of object VM
+Add the given value to the VCPUs/features/force\_off field of the given VM.  If the value is already in that Set, then do nothing.
 
  \noindent {\bf Signature:} 
 \begin{verbatim} void add_VCPUs_features_force_off (session_id s, VM ref self, cpu_feature value)\end{verbatim}
@@ -2803,7 +2803,7 @@ void
 \subsubsection{RPC name:~remove\_VCPUs\_features\_force\_off}
 
 {\bf Overview:} 
-set remove message derived from field VCPUs/features/force\_off of object VM
+Remove the given value from the VCPUs/features/force\_off field of the given VM.  If the value is not in that Set, then do nothing.
 
  \noindent {\bf Signature:} 
 \begin{verbatim} void remove_VCPUs_features_force_off (session_id s, VM ref self, cpu_feature value)\end{verbatim}