Add frame context line settings paragraph to Tuning & Overhead doc

Collecting frame context lines is quite expensive, especially for spans, we should mention the [settings](https://github.com/elastic/apm-agent-python/blob/master/docs/configuration.asciidoc#source_lines_error_app_frames) to configure the amount of collected lines in the [Tuning & Overhead](https://github.com/elastic/apm-agent-python/blob/master/docs/tuning.asciidoc) docs