docs suggestion- a linked "behavior" block for *_by_file directives

After reading about how require will only load something once per worker on the group, I wondered if the by_file would as well. It seems it does - but I had to jump through a lot of the docs to figure it out.

I wanted to suggest a small change to the docs.

These 3 sections all talk about this behavior:

• https://github.com/openresty/lua-nginx-module#data-sharing-within-an-nginx-worker
• https://github.com/openresty/lua-nginx-module#init_by_lua
• https://github.com/openresty/lua-nginx-module#lua_code_cache

And a handful of *_by_lua_file reference the lua_code_cache docs.

I think it might be a better reference guide if:

1. there was a narrative doc at the top like "How LUA-NGINX caches code and shares data" that consolidates all the info about this behavior
2. All the *_by_lua_file directives have a standard text that says something like:

Code caching is affected by lua_code_cache: Yes [or No]
For more information on code caching, see "How LUA-NGINX caches code"

Start by comparing the three linked sections: data sharing within an NGINX worker, init_by_lua, and lua_code_cache. Review the *_by_lua_file references to lua_code_cache and document the shared caching behavior in one narrative section, with consistent references from each directive. Done means the behavior is consolidated and all relevant directives point to it.