docs: add comment explaining vsn charlist format

- Document that Application.spec/2 returns :vsn as a charlist
- Clarify that string interpolation handles the conversion automatically
- Ensures version is always returned in proper semver string format

Co-authored-by: Claude Code <noreply@anthropic.com>

Author: emyller

lib/flagsmith_client.ex

@@ -11,6 +11,8 @@ defmodule Flagsmith.Client do
   @doc false
   @spec user_agent() :: String.t()
   def user_agent do
+    # Application.spec/2 returns :vsn as a charlist (e.g., ~c"2.2.1")
+    # String interpolation automatically converts it to the proper string format
     case Application.spec(:flagsmith_engine, :vsn) do
       nil -> "flagsmith-elixir-sdk/unknown"
       vsn -> "flagsmith-elixir-sdk/#{vsn}"