doc page clarifications

doc/src/fix_shake.txt

@@ -214,8 +214,10 @@ which can lead to poor energy conservation.  You can test for this in
 your system by running a constant NVE simulation with a particular set
 of SHAKE parameters and monitoring the energy versus time.
 
-SHAKE or RATTLE should not be used to constrain an angle at 180 degrees
-(e.g. linear CO2 molecule).  This causes numeric difficulties.
+SHAKE or RATTLE should not be used to constrain an angle at 180
+degrees (e.g. linear CO2 molecule).  This causes numeric difficulties.
+You can use "fix rigid or fix rigid/small"_fix_rigid.html instead to
+make a linear molecule rigid.
 
 [Related commands:] none
 

doc/src/minimize.txt

@@ -216,10 +216,10 @@ The "fix box/relax"_fix_box_relax.html command can be used to apply an
 external pressure to the simulation box and allow it to shrink/expand
 during the minimization.
 
-Only a few other fixes (typically those that apply force constraints)
-are invoked during minimization.  See the doc pages for individual
-"fix"_fix.html commands to see which ones are relevant.  Current
-examples of fixes that can be used include:
+Only a few other fixes (typically those that add forces) are invoked
+during minimization.  See the doc pages for individual "fix"_fix.html
+commands to see which ones are relevant.  Current examples of fixes
+that can be used include:
 
 "fix addforce"_fix_addforce.html
 "fix addtorque"_fix_addtorque.html
@@ -242,6 +242,11 @@ you MUST enable the "fix_modify"_fix_modify.html {energy} option for
 that fix.  The doc pages for individual "fix"_fix.html commands
 specify if this should be done.
 
+NOTE: The minimizers in LAMMPS do not allow for bonds (or angles, etc)
+to be held fixed while atom coordinates are being relaxed, e.g. via
+"fix shake"_fix_shake.html or "fix rigid"_fix_rigid.html.  See more
+info in the Restrictions section below.
+
 :line
 
 [Restrictions:]