Document clear_cache in the plotting API reference

[goelakash]

clear_cache is exported from the package in yabplot/__init__.py alongside the six plot_* functions, but it is the only one of those seven with no entry in docs/reference/plotting.md, so it never appears in the built API reference.

Added it using the same show_root_heading block as the others. It already has a docstring, so nothing else was needed.

Checked the other five reference pages the same way: every symbol they document exists, and clear_cache was the only public export missing a page.