Explain languages chart calculation

The chart showing "Programming languages used in this repository" did
not explain how the chart was calculated. This adds a brief
explanation.

app/views/projects/graphs/charts.html.haml

@@ -4,6 +4,9 @@
   %h4.sub-header
     = _("Programming languages used in this repository")
 
+  %p
+    = _("Measured in bytes of code. Excludes generated and vendored code.")
+
   .row
     .col-md-4
       %ul.bordered-list

changelogs/unreleased/djensen-explain-programming-languages-chart.yml

+---
+title: Add measurement details for programming languages graph
+merge_request: 20592
+author:
+type: changed

locale/gitlab.pot

@@ -11118,6 +11118,9 @@ msgstr ""
 msgid "May"
 msgstr ""
 
+msgid "Measured in bytes of code. Excludes generated and vendored code."
+msgstr ""
+
 msgid "Median"
 msgstr ""