Added missing doc for plumedcheck astyle

src/maketools/plumedcheck

@@ -198,6 +198,13 @@ BEGINFILE{
     tempfile = tmpdir "/astyle"
     system(astyle " --options=" astyle_options " < " FILENAME " > " tempfile)
     s=system("diff -q " FILENAME " " tempfile ">/dev/null 2>/dev/null")
+# check if astyle has been applied correctly
+# DOC: :astyle:
+# DOC: In order to keep the code readable, we use the astyle program to format
+# DOC: all source files. This error indicates that the reported file has not been
+# DOC: formatted correctly.  It is important that the version of astyle and the options
+# DOC: exactly match the ones we use for testing. To this aim, you should use
+# DOC: a command such as `make astyle` from the plumed root directory.
     if(s!=0) error("astyle","astyle not satisfied")
     system("rm " tempfile)
   }