rc: autogenerate and tidy the docs and commands

* Rename rc/pid -> core/pid
  * Sort the output of `rc list`
  * Make a script to autogenerate the docs
  * Tidy docs

docs/content/rc.md

@@ -68,6 +68,22 @@ Run `rclone rc` on its own to see the help for the installed remote
 control commands.
 
 ## Supported commands
+<!--- autogenerated start - run make rcdocs - don't edit here -->
+### cache/expire: Purge a remote from cache
+
+Purge a remote from the cache backend. Supports either a directory or a file.
+Params:
+  - remote = path to remote (required)
+  - withData = true/false to delete cached data (chunks) as well (optional)
+
+Eg
+
+    rclone rc cache/expire remote=path/to/sub/folder/
+    rclone rc cache/expire remote=/ withData=true
+
+### cache/stats: Get cache stats
+
+Show statistics for the cache remote.
 
 ### core/bwlimit: Set the bandwidth limit.
 
@@ -78,18 +94,41 @@ Eg
     rclone rc core/bwlimit rate=1M
     rclone rc core/bwlimit rate=off
 
-### cache/expire: Purge a remote from cache
+The format of the parameter is exactly the same as passed to --bwlimit
+except only one bandwidth may be specified.
 
-Purge a remote from the cache backend. Supports either a directory or a file.
-Params:
+### core/memstats: Returns the memory statistics
 
-  - remote = path to remote (required)
-  - withData = true/false to delete cached data (chunks) as well (optional)
+This returns the memory statistics of the running program.  What the values mean
+are explained in the go docs: https://golang.org/pkg/runtime/#MemStats
 
-Eg
+The most interesting values for most people are:
 
-    rclone rc cache/expire remote=path/to/sub/folder/
-    rclone rc cache/expire remote=/ withData=true 
+* HeapAlloc: This is the amount of memory rclone is actually using
+* HeapSys: This is the amount of memory rclone has obtained from the OS
+* Sys: this is the total amount of memory requested from the OS
+  * It is virtual memory so may include unused memory
+
+### core/pid: Return PID of current process
+
+This returns PID of current process.
+Useful for stopping rclone process.
+
+### rc/error: This returns an error
+
+This returns an error with the input as part of its error string.
+Useful for testing error handling.
+
+### rc/list: List all the registered remote control commands
+
+This lists all the registered remote control commands as a JSON map in
+the commands response.
+
+### rc/noop: Echo the input to the output parameters
+
+This echoes the input parameters to the output parameters for testing
+purposes.  It can be used to check that rclone is still alive and to
+check that parameter passing is working properly.
 
 ### vfs/forget: Forget files or directories in the directory cache.
 
@@ -107,26 +146,7 @@ starting with dir will forget that dir, eg
 
     rclone rc vfs/forget file=hello file2=goodbye dir=home/junk
 
-### rc/noop: Echo the input to the output parameters
-
-This echoes the input parameters to the output parameters for testing
-purposes.  It can be used to check that rclone is still alive and to
-check that parameter passing is working properly.
-
-### rc/error: This returns an error
-
-This returns an error with the input as part of its error string.
-Useful for testing error handling.
-
-### rc/pid: Return PID of current process
-
-This returns PID of current process.
-Useful for stopping rclone process.
-
-### rc/list: List all the registered remote control commands
-
-This lists all the registered remote control commands as a JSON map in
-the commands response.
+<!--- autogenerated stop -->
 
 ## Accessing the remote control via HTTP