Improve composite docs and changelog.

PiperOrigin-RevId: 730394496
Change-Id: Ie993214deb82b526a96510e02f6ecff103c58607

src/user/user_composite.cc

@@ -58,7 +58,6 @@ mjCComposite::mjCComposite(void) {
   type = mjCOMPTYPE_PARTICLE;
   count[0] = count[1] = count[2] = 1;
   mjuu_setvec(offset, 0, 0, 0);
-  flatinertia = 0;
 
   // plugin variables
   mjs_defaultPlugin(&plugin);

src/user/user_composite.h

@@ -38,11 +38,6 @@ typedef enum _mjtCompType {
 
 typedef enum _mjtCompKind {
   mjCOMPKIND_JOINT = 0,
-  mjCOMPKIND_TWIST,
-  mjCOMPKIND_STRETCH,
-  mjCOMPKIND_TENDON,
-  mjCOMPKIND_SHEAR,
-  mjCOMPKIND_PARTICLE,
 
   mjNCOMPKINDS
 } mjtCompKind;
@@ -79,7 +74,6 @@ class mjCComposite {
   mjtCompType type;               // composite type
   int count[3];                   // geom count in each dimension
   double offset[3];               // position offset for particle and grid
-  double flatinertia;             // flatten ineria of cloth elements; 0: disable
 
   // currently used only for cable
   std::string initial;            // root boundary type