docs: layout tweak of caching info, remove toolchain :( and additiona…

…l info about bazel-remote-cache (#19299)

Minor doc improvements.

docs/markdown/Using Pants/remote-caching-execution.md

@@ -20,7 +20,11 @@ What is Remote Execution API?
 
 Pants is compatible with remote caching and remote execution servers that comply with the [Remote Execution API](https://github.com/bazelbuild/remote-apis) standard ("REAPI"). The REAPI protocol is supported by several different server and client projects including Bazel and of course Pants.
 
-REAPI servers implement several related but distinct services: (1) a "content-addressable storage" service that stores data keyed by the hash of that data (also known as a "CAS"); (2) an "action cache service" that maps process executions to their results; and (3) an "execution service" that executes processes by using the content-addressable storage service to obtain the inputs and store the outputs from running those processes.
+REAPI servers implement several related but distinct services:
+
+1. A "content-addressable storage" service that stores data keyed by the hash of that data (also known as a "CAS").
+2. An "action cache service" that maps process executions to their results.
+3. An "execution service" that executes processes by using the content-addressable storage service to obtain the inputs and store the outputs from running those processes.
 
 Remote cache servers implement the CAS and action cache services. Remote execution servers implement all three services.
 
@@ -33,17 +37,21 @@ Server compatibility
 
 In order to use remote caching or remote execution, Pants will need access to a server that complies with REAPI. Pants is known to work with:
 
-**SaaS**:
-- [Toolchain](https://www.toolchain.com), a remote caching service designed by several of the lead maintainers of Pants specifically to work seamlessly with it.
-
 **Self-hosted**:
 - [BuildBarn](https://github.com/buildbarn/bb-remote-execution)
 - [Buildfarm](https://github.com/bazelbuild/bazel-buildfarm/) 
 - [BuildGrid](https://buildgrid.build/)
-- [bazel-remote](https://github.com/buchgr/bazel-remote]
 
 **Note**: Setup of a self-hosted REAPI server is beyond the scope of this documentation. All these server projects have support channels on the BuildTeamWorld Slack. [Go here to obtain an invite to that Slack.](https://bit.ly/2SG1amT)
 
+As a more lightweight solution - for caching only - Pants can use a server providing only the CAS (content-addressable storage) service.
+
+**Self-hosted**:
+
+- [bazel-remote-cache](https://github.com/buchgr/bazel-remote)
+
+Bazel Remote Cache supports local disk, S3, GCS, and Azure Blob storage options - needing only a single lightweight Docker container.
+
 There are a few [other](https://github.com/bazelbuild/remote-apis) systems and services in this space, but
 they have not, to our knowledge, been tested with Pants.  Let us know if you have any experience with them!