Add documentation for s parameter

docs/data_search.md

@@ -431,3 +431,24 @@ For example if the user wants all observations in csv format:
 ```bash
 curl "http://localhost:8080/search/observations?a=1;2;3&f=csv"
 ```
+
+## Search trajectory parameter (param s: not of use for a search)
+
+There is, for the client, an optional parameter `s`. The latter is used to check
+on which step, the user is passed on `criteria`, `output` or `result` during a search through a dataset (1 if
+passed, 0 if not). 
+
+For instance, if the user passed through all of them while searching a dataset,
+you will see in the search bar something like:
+```
+"http://localhost:4200/instance/default/search/result/observed_spectra?s=111&a=1;3;4;5;7;8;9;11"
+```
+
+If you stopped at the result table, you will see:
+```
+"http://localhost:4200/instance/default/search/result/observed_spectra?s=110&a=1;3;4;5;7;8;9;11"
+```
+
+This parameter is _not of use for a search, and can be omitted safely_ while
+performing a query. We mention it here, for the sake of completeness and
+clarity.