sso_proxy: document provider slug configs

[Jusshersmith]

Problem

Some documentation for the upstream configuration option provider_slug, and the global sso_proxy
DefaultProviderSlug option is missing

Solution

Add in the missing documentation.

[codecov]

Codecov Report

Merging #301 into master will decrease coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #301      +/-   ##
==========================================
- Coverage   61.98%   61.84%   -0.15%     
==========================================
  Files          57       57              
  Lines        4646     4188     -458     
==========================================
- Hits         2880     2590     -290     
+ Misses       1554     1388     -166     
+ Partials      212      210       -2     

Impacted Files	Coverage Δ
internal/proxy/options.go	83.59% <ø> (-1.08%)	⬇️
internal/auth/http.go	60.00% <0.00%> (-6.67%)	⬇️
internal/auth/providers/internal_util.go	55.55% <0.00%> (-4.45%)	⬇️
internal/auth/static_files.go	69.23% <0.00%> (-4.11%)	⬇️
internal/auth/configuration.go	43.68% <0.00%> (-3.74%)	⬇️
internal/pkg/aead/aead.go	62.96% <0.00%> (-3.71%)	⬇️
internal/auth/circuit/breaker.go	81.98% <0.00%> (-2.40%)	⬇️
internal/pkg/groups/fillcache.go	72.30% <0.00%> (-2.35%)	⬇️
internal/proxy/proxy_config.go	76.21% <0.00%> (-2.28%)	⬇️
internal/auth/middleware.go	80.43% <0.00%> (-2.26%)	⬇️
... and 46 more

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[jphines]

This is annoying. Do we want to force merge?

[Jusshersmith]

@jphines Actually, I think that might be a good idea. I've tried to dig into why it's suggesting this change in code coverage, but unclear at the moment.

If you're happy with the change, let's force merge 👍