[doc] misc fix & improvements

[doc] misc fix & improvements

  * insertion, extraction: Main <variablelist> replaced with <refsect3>
                           No content changed (the diff is far bad)
  * gsort: rank of "" when sorting text
  * polyline_properties: image of line styles fixed
  * cumsum: + warning: sparse density => 100%: http://bugzilla.scilab.org/8100
            + completing http://bugzilla.scilab.org/14297 for cumsum
            + <latex alt=".."> added and other improvements
  * mtlb_sparse: no longer exists: http://bugzilla.scilab.org/14477
  * symbols: ** deprecated equivalent of ^:http://bugzilla.scilab.org/16568
  * xsave: remark @ http://mailinglists.scilab.org/Scilab-users-xsave-or-save-tt4041024.html
  * replot(): note added: gca() stays the same.
  * example_run: description of moduleNames improved (for external modules)
                 This documents the change https://codereview.scilab.org/8698
  * percent: wrong overload name fixed in example. See also improved.
             SCI/modules/overloading/macros/README no longer exists.
  * bode: example #3 plot: avoid extra tight x limit

Change-Id: Ia8a083ba7f80f7e91bf4f2e75ef92c0a74121486