docs: update rclone about docs

Create a full loop of documentation for rclone about, backends overview
and individual backend pages.

Discussion:
https://github.com/rclone/rclone/pull/4774 relates

Previously pull was requested, in part, under ref
https://github.com/rclone/rclone/pull/4801

Notes:
Introduce a tentative draft see-link format the end of sections to try
rather than lots of in-para links.

Update about.go incl link to list of backends not supporting about.

In list of backends not supporting about, include link to about command
reference.

I appreciate there may be decisions to make going forward about whether
command links should be code formatted, and using proper pretty url
links, but I have fudged that for now.

Update backend pages that do not support about with wording used
previously for ftp - it is in passive voice but I can live with it. (my
own wording and fault). The note is applied to a limitations section. If
one does not already exist it is created (even if there are other
limitations with their own sections)

docs/content/azureblob.md

@@ -327,12 +327,19 @@ See: the [encoding section in the overview](/overview/#encoding) for more info.
 - Default:     Slash,BackSlash,Del,Ctl,RightPeriod,InvalidUtf8
 
 {{< rem autogenerated options stop >}}
-
 ### Limitations ###
 
 MD5 sums are only uploaded with chunked files if the source has an MD5
 sum.  This will always be the case for a local to azure copy.
 
+`rclone about` is not supported by the Microsoft Azure Blob storage backend. Backends without
+this capability cannot determine free space for an rclone mount or
+use policy `mfs` (most free space) as a member of an rclone union
+remote.
+
+See [List of backends that do not support rclone about](https://rclone.org/overview/#optional-features)
+See [rclone about](https://rclone.org/commands/rclone_about/)
+
 ### Azure Storage Emulator Support ###
 You can test rclone with storage emulator locally, to do this make sure azure storage emulator
 installed locally and set up a new remote with `rclone config` follow instructions described in