Documentation issue: Remote caching

Documentation URL: https://docs.bazel.build/versions/master/remote-caching.html

Where is the documentation for how to provide bazel options to use grpcs as a remote cache? In particular, what formats are the keys and certs supposed to be in? I used:

```
bazel build \
--remote_cache=grpcs://192.0.2.42:8086 \
--tls_certificate=ca.pem \
--tls_client_certificate=client.pem \
--tls_client_key=client-key.pem \
//...
```
and got a complaint about the client key being in the wrong format. Some searching revealed that it should have been in PKCS#8 format, so I converted it and ran:
```
bazel build \
--remote_cache=grpcs://192.0.2.42:8086 \
--tls_certificate=ca.pem \
--tls_client_certificate=client.pem \
--tls_client_key=client-key.p8 \
//...
```
Which gave me the singularly unenlightening message: `ERROR: Failed to query remote execution capabilities: General OpenSslEngine problem` (I guess saying what the problem was is too much effort?)

The help message isn't enlightening either:
```
$ bazel help build | grep tls_client_key
--tls_client_key (a string; default: see description)
```
*What* description?

Start with the remote caching documentation at https://docs.bazel.build/versions/master/remote-caching.html and inspect the descriptions for --tls_certificate, --tls_client_certificate, and --tls_client_key. Document the accepted certificate and key formats, the relevant option descriptions, and enough troubleshooting guidance for the reported OpenSSL error; done when users can configure grpcs remote caching without searching elsewhere.

Written by the indexing model from the issue text.