Document the optional parameter of the cd command

README.asciidoc

@@ -620,7 +620,7 @@ Some commands take an exclamation mark (`!`), which can be used to force
 the execution of the command (i.e. to quit a modified buffer, the
 command `q!` has to be used).
 
- * `cd <directory>`: change the current directory to the one passed
+ * `cd [<directory>]`: change the current directory to `<directory>`, or the home directory is unspecified
  * `e[dit][!] <filename> [<line> [<column>]]`: open buffer on file, go to given
      line and column. If file is already opened, just switch to this file.
      use edit! to force reloading.

doc/manpages/commands.asciidoc

@@ -12,8 +12,9 @@ Some commands take an exclamation mark (*!*), which can be used to force
 the execution of the command (i.e. to quit a modified buffer, the
 command *q!* has to be used).
 
-*cd* <directory>::
-	change the current directory to the one passed
+*cd* [<directory>]::
+	change the current directory to *directory*, or the home directory if
+	unspecified
 
 *e[dit][!]* <filename> [<line> [<column>]]::
 	open buffer on file, go to given line and column. If file is already

src/commands.cc

@@ -1958,7 +1958,7 @@ const CommandDesc select_cmd = {
 const CommandDesc change_working_directory_cmd = {
     "cd",
     nullptr,
-    "cd <dir>: change server working directory to <dir>",
+    "cd [<directory>]: change the server's working directory to <directory>, or the home directory if unspecified",
     single_optional_name_param,
     CommandFlags::None,
     CommandHelper{},