docs: add short description of configuration file format (#5142)

Fixes #572

docs/content/docs.md

@@ -8,8 +8,8 @@ Configure
 
 First, you'll need to configure rclone.  As the object storage systems
 have quite complicated authentication these are kept in a config file.
-(See the `--config` entry for how to find the config file and choose
-its location.)
+(See the [`--config`](#config-config-file) entry for how to find the config
+file and choose its location.)
 
 The easiest way to make the config is to run rclone with the config
 option:
@@ -639,7 +639,7 @@ See `--copy-dest` and `--backup-dir`.
 
 ### --config=CONFIG_FILE ###
 
-Specify the location of the rclone config file.
+Specify the location of the rclone configuration file.
 
 Normally the config file is in your home directory as a file called
 `.config/rclone/rclone.conf` (or `.rclone.conf` if created with an
@@ -656,8 +656,44 @@ location is for you.
 Use this flag to override the config location, e.g. `rclone
 --config=".myconfig" .config`.
 
-If this is set to empty string or the special value `/notfound` then
-rclone will keep the config file in memory only.
+If the location is set to empty string `""` or the special value
+`/notfound`, or the os null device represented by value `NUL` on
+Windows and `/dev/null` on Unix systems, then rclone will keep the
+config file in memory only.
+
+The file format is basic [INI](https://en.wikipedia.org/wiki/INI_file#Format):
+Sections of text, led by a `[section]` header and followed by
+`key=value` entries on separate lines. In rclone each remote is
+represented by its own section, where the section name defines the
+name of the remote. Options are specified as the `key=value` entries,
+where the key is the option name without the `--backend-` prefix,
+in lowercase and with `_` instead of `-`. E.g. option `--mega-hard-delete`
+corresponds to key `hard_delete`. Only backend options can be specified.
+A special, and required, key `type` identifies the [storage system](/overview/),
+where the value is the internal lowercase name as returned by command
+`rclone help backends`. Comments are indicated by `;` or `#` at the
+beginning of a line.
+
+Example:
+
+    [megaremote]
+    type = mega
+    user = you@example.com
+    pass = PDPcQVVjVtzFY-GTdDFozqBhTdsPg3qH
+
+Note that passwords are in [obscured](/commands/rclone_obscure/)
+form. Also, many storage systems uses token-based authentication instead
+of passwords, and this requires additional steps. It is easier, and safer,
+to use the interactive command `rclone config` instead of manually
+editing the configuration file.
+
+The configuration file will typically contain login information, and
+should therefore have restricted permissions so that only the current user
+can read it. Rclone tries to ensure this when it writes the file.
+You may also choose to [encrypt](#configuration-encryption) the file.
+
+When token-based authentication are used, the configuration file
+must be writable, because rclone needs to update the tokens inside it.
 
 ### --contimeout=TIME ###