[DOC] Add explanation to PATTERN of brackets

Fixes: #1933

doc/rofi.1

@@ -1340,6 +1340,15 @@ replaced at runtime:
 
 .RE
 
+.PP
+It processes the string as follows: \fB\fC{key}\fR
+is replaced by its value, if \fB\fC{key}\fR is not set it is removed. If the \fB\fC{key}\fR
+is in between \fB\fC[]\fR  all the text between \fB\fC[]\fR is removed if \fB\fC{key}\fR is not set.
+Otherwise key is replaced and the \fB\fC[]\fR are removed.
+
+.PP
+For example: \fB\fC{ssh-client} [-p {port}] {host}\fR
+
 .SH THEMING
 .PP
 Please see \fBrofi-theme(5)\fP manpage for more information on theming.

doc/rofi.1.markdown

@@ -880,6 +880,13 @@ replaced at runtime:
 - `{cmd}`: the command to execute
 - `{window}`: the window ID of the selected window (in `window-command`)
 
+It processes the string as follows: `{key}`
+is replaced by its value, if `{key}` is not set it is removed. If the `{key}`
+is in between `[]`  all the text between `[]` is removed if `{key}` is not set.
+Otherwise key is replaced and the `[]` are removed.
+
+For example: `{ssh-client} [-p {port}] {host}`
+
 ## THEMING
 
 Please see **rofi-theme(5)** manpage for more information on theming.