Add keepalive_expiry to Limits config

[tomchristie]

Closes #1395

We do also want #1302, but I think it makes sense to include keepalive_expiry in the Limits config either ways around.

[florimondmanca]

Nice and simple enough, yes. :-)

Don't think we have it yet, but it strikes me that a "Pooling configuration" section in the advanced docs could be a good contribution, and this new option could easily be showcased there.

[tomchristie]

@florimondmanca Yeah that'd be a good one. TBH I think we could really do with completely overhauling our docs structure. We've mirrored requests so far, but actually I could see us doing better with more sections, and going into depth in each.

URLs, Headers, Request/Response content, Cookies, Authentication, Concurrency, Proxies, SSL, HTTP/2, Connection Pooling, etc...

Having fuller sections for each would also let us fill them out in more depth with good learning material, whereas when everything follows on in the advanced section we kinda have to keep each bit relatively brief.

[florimondmanca]

@tomchristie Yes, I also feel like our docs could use some overhaul and restructuring... As always, docs from trio projects like trio and h11 are great inspiration sources on what could be. :) Tutorials for common problems and highlighting some best practices or common gotchas, in-depth guides about various pieces of functionality, etc.