docs: add a section about measurement tables

Author: ManonMarchand

docs/simbad/simbad.rst

@@ -602,6 +602,72 @@ And the columns in the output can be reset to their default value with
     A detailed description on the ways to add fluxes is available in the
     :ref:`optical filters` section.
 
+Measurement fields vs. Basic fields
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Some field names start with ``mes``. These denote what SIMBAD calls a
+"measurement table". These tables store the history on past measurements of a physical
+parameter for each object.
+
+Let's look at the star ``HD 200207`` with the parallax measurements table ``mesplx``:
+
+.. doctest-remote-data::
+
+    >>> from astroquery.simbad import Simbad
+    >>> simbad = Simbad()
+    >>> simbad.add_votable_fields("mesplx")
+    >>> hd_200207 = simbad.query_object("HD 200207")
+    >>> hd_200207[["main_id", "mesplx.plx", "mesplx.plx_err", "mesplx.bibcode"]]
+    <Table length=5>
+     main_id  mesplx.plx mesplx.plx_err    mesplx.bibcode
+                 mas          mas
+      object   float32      float32            object
+    --------- ---------- -------------- -------------------
+    HD 200207     3.4084         0.0195 2020yCat.1350....0G
+    HD 200207     3.4552         0.0426 2018yCat.1345....0G
+    HD 200207       3.35           0.76 1997A&A...323L..49P
+    HD 200207       3.72           0.62 2007A&A...474..653V
+    HD 200207       3.25           0.22 2016A&A...595A...2G
+
+This field adds one line per parallax measurement: five articles have measured it
+for this star.
+
+If you are only interested in the most precise measure recorded by the SIMBAD team, some
+measurements fields have an equivalent in the basic fields. These fields only give one
+line per object with the most precise currently known value:
+
++-------------------+---------------+
+| measurement field | basic field   |
++===================+===============+
+| mesplx            | parallax      |
++-------------------+---------------+
+| mespm             | propermotions |
++-------------------+---------------+
+| messpt            | sp            |
++-------------------+---------------+
+| mesvelocities     | velocity      |
++-------------------+---------------+
+
+Here, ``mesplx`` has an equivalent in the basic fields so we could have done:
+
+.. doctest-remote-data::
+
+    >>> from astroquery.simbad import Simbad
+    >>> simbad = Simbad()
+    >>> simbad.add_votable_fields("parallax")
+    >>> hd_200207 = simbad.query_object("HD 200207")
+    >>> hd_200207[["main_id", "plx_value", "plx_err", "plx_bibcode"]]
+    <Table length=1>
+     main_id  plx_value plx_err     plx_bibcode
+                 mas      mas
+      object   float64  float32        object
+    --------- --------- ------- -------------------
+    HD 200207    3.4084  0.0195 2020yCat.1350....0G
+
+And we only have one line per object with the value selected by SIMBAD's team.
+
+Thus, choosing to add a measurement field or a basic field depends on your goal.
+
 Additional criteria
 -------------------