Document about ${HOME} expansion

doc/manual-src/en/aria2c.rst

@@ -1958,6 +1958,43 @@ lines beginning ``#`` are treated as comments::
   bits of the configuration file (e.g., ``chmod 600 aria2.conf``), so
   that other user cannot see the contents of the file.
 
+The environment variables, such as ``${HOME}``, are expanded by shell.
+This means that those variables used in configuration file are not
+expanded.  However, it is useful to ``${HOME}`` to refer user's home
+directory in configuration file to specify file paths.  Therefore,
+aria2 expands ``${HOME}`` found in the following option values to
+user's home directory:
+
+* :option:`ca-certificate <--ca-certificate>`
+* :option:`certificate <--certificate>`
+* :option:`dht-file-path <--dht-file-path>`
+* :option:`dht-file-path6 <--dht-file-path6>`
+* :option:`dir <--dir>`
+* :option:`input-file <--input-file>`
+* :option:`load-cookies <--load-cookies>`
+* :option:`log <--log>`
+* :option:`metalink-file <--metalink-file>`
+* :option:`netrc-path <--netrc-path>`
+* :option:`on-bt-download-complete <--on-bt-download-complete>`
+* :option:`on-download-complete <--on-download-complete>`
+* :option:`on-download-error <--on-download-error>`
+* :option:`on-download-start <--on-download-start>`
+* :option:`on-download-stop <--on-download-stop>`
+* :option:`on-download-pause <--on-download-pause>`
+* :option:`out <--out>`
+* :option:`private-key <--private-key>`
+* :option:`rpc-certificate <--rpc-certificate>`
+* :option:`rpc-private-key <--rpc-private-key>`
+* :option:`save-cookies <--save-cookies>`
+* :option:`save-session <--save-session>`
+* :option:`server-stat-if <--server-stat-if>`
+* :option:`server-stat-of <--server-stat-of>`
+* :option:`torrent-file <--torrent-file>`
+
+Note that this expansion occurs even if the above options are used in
+the command-line.  This means that expansion may occur 2 times: first,
+shell and then aria2c.
+
 dht.dat
 ~~~~~~~~