xs2* doc: add that these functions might need an increased java heap size

the suggestion is to include the following note in each help page to work around memory limitation issue that may occur during the export. <note> In case <function>xs2ps</function> is issuing an error message complaining about not enough memory, you may manage this situation by iteratively : reducing the number of graphical elements visible in the figure (zoom level adjust) and increasing the java heap size (java heap size limit is set by default to 512 Mo under Scilab console Menu Edit > Preferences > Generalities. NB: restart Scilab is required to apply the new heap size), until you reach the correct zoom level and successul export file in vectorized format. </note>