bug 1496789 - Document the limits for n_buckets and n_values r=Dexter

MozReview-Commit-ID: CxklgZ8VPyN

Differential Revision: https://phabricator.services.mozilla.com/D8902

--HG--
extra : moz-landing-system : lando

toolkit/components/telemetry/docs/collection/histograms.rst

@@ -168,10 +168,18 @@ Required for linear and exponential histograms. The maximum value to be stored i
 -------------
 Required for linear and exponential histograms. The number of buckets in a linear or exponential histogram.
 
+.. note::
+
+    The maximum value for ``n_buckets`` is 100. The more buckets, the larger the storage and transfer costs borne by our users and our pipeline.
+
 ``n_values``
 ------------
 Required for enumerated histograms. Similar to n_buckets, it represent the number of elements in the enum.
 
+.. note::
+
+    The maximum value for ``n_values`` is 100. The more values, the larger the storage and transfer costs borne by our users and our pipeline.
+
 ``labels``
 ----------
 Required for categorical histograms. This is an array of strings which are the labels for different values in this histograms. The labels are restricted to a C++-friendly subset of characters (``^[a-z][a-z0-9_]+[a-z0-9]$``). This field is limited to 100 strings, each with a maximum length of 20 characters.