eslint [nfc]: Add comments explaining each line of .eslintignore.

Also discussing a bug in the interaction of `tools/test` with `eslint`
that appears when you're on a branch that's touched one of the
ignored files.

.eslintignore

@@ -1,2 +1,18 @@
+# If you run `tools/test` when you've modified a file that's ignored
+# here, you might get a warning like this:
+#   0:0  warning  File ignored because of a matching ignore pattern. […]
+#
+# Don't worry about that warning; it won't appear in CI, and it won't
+# appear on future `tools/test` runs when not editing these files.
+# For more discussion, see:
+#   https://github.com/zulip/zulip-mobile/pull/4430#issuecomment-767297445
+
+# These are purely type definitions, no runtime code.  Most of them
+# are third-party code, too, so naturally don't match our style.
 **/flow-typed/**
-**/__flow-tests__/**
+
+# These are type-tests, made up of code that gets type-checked but
+# never actually run.  They're naturally full of dead code which
+# ESLint would complain about; and because the code never runs, other
+# things it might complain about don't really matter anyway.
+**/__flow-tests__/**