linter: Add check for comma after "e.g." in md files.

Adds a linter rule for markdown files so that new cases of "e.g."
without a comma wont be introduced to user-facing documentation.

Fixes the remaining cases of "e.g." in markdown files that are not
followed by a comma.

tools/linter_lib/custom_check.py

@@ -879,6 +879,10 @@ markdown_rules = RuleList(
             "include_only": {"docs/"},
             "description": "Don't link directly to line numbers",
         },
+        {
+            "pattern": r"[eE]\.g\.[^,]",
+            "description": "Likely missing comma after 'e.g.'",
+        },
     ],
 )