rename noextra option to nofix

since this corresponds to the `fix` option of `read_data`, calling this flag `nofix` seems more consistent and descriptive.

doc/src/write_data.txt

@@ -16,7 +16,7 @@ file = name of data file to write out :ulb,l
 zero or more keyword/value pairs may be appended :l
 keyword = {pair} or {nocoeff} :l
   {nocoeff} = do not write out force field info
-  {noextra} = do not write out extra sections
+  {nofix} = do not write out extra sections read by fixes
   {pair} value = {ii} or {ij}
     {ii} = write one line of pair coefficient info per atom type
     {ij} = write one line of pair coefficient info per IJ atom type pair :pre
@@ -84,9 +84,11 @@ be written to the data file. This can be very helpful, if one wants
 to make significant changes to the force field or if the parameters
 are read in separately anyway, e.g. from an include file.
 
-The {noextra} keyword requests that no extra sections should be
-written to the data file. For example, this option excludes sections
-for user-created per-atom properties, which are included by default.
+The {nofix} keyword requests that no extra sections read by fixes
+should be written to the data file (see the {fix} option of the
+"read_data"_read_data.html command for details). For example, this
+option excludes sections for user-created per-atom properties 
+from "fix property/atom"_fix_property_atom.html.
 
 The {pair} keyword lets you specify in what format the pair
 coefficient information is written into the data file.  If the value