doc: Update the document of `txprepare`

doc/lightning-txprepare.7

@@ -3,17 +3,33 @@
 lightning-txprepare - Command to prepare to withdraw funds from the internal wallet
 .SH SYNOPSIS
 
-\fBtxprepare\fR \fIdestination\fR \fIsatoshi\fR [\fIfeerate\fR] [\fIminconf\fR]
+\fBtxprepare\fR \fIoutputs\fR [\fIfeerate\fR] [\fIminconf\fR]
 
 .SH DESCRIPTION
 
 The \fBtxprepare\fR RPC command creates an unsigned transaction which
-spends funds from c-lightning’s internal wallet to the address specified
-in \fIdestination\fR\.
-
-
-Effectively, it is the first part of a \fBwithdraw\fR command, and uses
-the same parameters\. The second part is provided by \fBtxsend\fR\.
+spends funds from c-lightning’s internal wallet to the outputs specified
+in \fIoutputs\fR\.
+
+The \fIoutputs\fR is the array of output that include \fIdestination\fR
+and \fIamount\fR({\fIdestination\fR: \fIamount\fR})\. Its format is like:
+[{address1: amount1}, {address2: amount2}]
+or
+[{address: \fIall\fR}]\.
+It supports the any number of outputs\.
+
+The \fIdestination\fR of output is the address which can be of any Bitcoin accepted
+type, including bech32\.
+
+The \fIamount\fR of output is the amount to be sent from the internal wallet
+(expressed, as name suggests, in amount)\. The string \fIall\fR can be used to specify
+all available funds\. Otherwise, it is in amount precision; it can be a whole
+number, a whole number ending in \fIsat\fR, a whole number ending in \fI000msat\fR,
+or a number with 1 to 8 decimal places ending in \fI000msat\fR\.
+
+\fBtxprepare\fR is similar to the first part of a \fBwithdraw\fR command, but
+supports multiple outputs and uses \fIoutputs\fR as parameter\. The second part
+is provided by \fBtxsend\fR\.
 
 .SH RETURN VALUE
 

doc/lightning-txprepare.7.md

@@ -4,17 +4,34 @@ lightning-txprepare -- Command to prepare to withdraw funds from the internal wa
 SYNOPSIS
 --------
 
-**txprepare** *destination* *satoshi* \[*feerate*\] \[*minconf*\]
+**txprepare** *outputs* \[*feerate*\] \[*minconf*\]
 
 DESCRIPTION
 -----------
 
 The **txprepare** RPC command creates an unsigned transaction which
-spends funds from c-lightning’s internal wallet to the address specified
-in *destination*.
-
-Effectively, it is the first part of a **withdraw** command, and uses
-the same parameters. The second part is provided by **txsend**.
+spends funds from c-lightning’s internal wallet to the outputs specified
+in *outputs*.
+
+The *outputs* is the array of output that include *destination*
+and *amount*(\{*destination*: *amount*\}). Its format is like:
+\[\{address1: amount1\}, \{address2: amount2\}\]
+or
+\[\{address: *all*\}\].
+It supports the any number of outputs.
+
+The *destination* of output is the address which can be of any Bitcoin accepted
+type, including bech32.
+
+The *amount* of output is the amount to be sent from the internal wallet
+(expressed, as name suggests, in amount). The string *all* can be used to specify
+all available funds. Otherwise, it is in amount precision; it can be a whole
+number, a whole number ending in *sat*, a whole number ending in *000msat*,
+or a number with 1 to 8 decimal places ending in *btc*.
+
+**txprepare** is similar to the first part of a **withdraw** command, but
+supports multiple outputs and uses *outputs* as parameter. The second part
+is provided by **txsend**.
 
 RETURN VALUE
 ------------