doc: add documentation for invoicerequest commands.

As reported on Discord, these are undocumented.  And thus, um, hard to find!

Reported-by: Aaron Barnard
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/lightning-disableinvoicerequest.7.md

@@ -0,0 +1,54 @@
+lightning-disableinvoicerequest -- Command for removing an invoice request
+==========================================================================
+
+SYNOPSIS
+--------
+**(WARNING: experimental-offers only)**
+
+**disableinvoicerequest** *invreq\_id*
+
+DESCRIPTION
+-----------
+
+The **disableinvoicerequest** RPC command disables an
+invoice\_request, so that no further invoices will be accepted (and
+thus, no further payments made)..
+
+We currently don't support deletion of invoice\_requests, so they are
+not forgotten entirely (there may be payments which refer to this
+invoice\_request).
+
+
+RETURN VALUE
+------------
+
+Note: the returned object is the same format as **listinvoicerequest**.
+
+[comment]: # (GENERATE-FROM-SCHEMA-START)
+On success, an object is returned, containing:
+
+- **invreq\_id** (hash): the SHA256 hash of all invoice\_request fields less than 160
+- **active** (boolean): whether the invoice\_request is currently active (always *false*)
+- **single\_use** (boolean): whether the invoice\_request will become inactive after we pay an invoice for it
+- **bolt12** (string): the bolt12 string starting with lnr
+- **used** (boolean): whether the invoice\_request has already been used
+- **label** (string, optional): the label provided when creating the invoice\_request
+
+[comment]: # (GENERATE-FROM-SCHEMA-END)
+
+AUTHOR
+------
+
+Rusty Russell <<rusty@rustcorp.com.au>> is mainly responsible.
+
+SEE ALSO
+--------
+
+lightning-invoicerequest(7), lightning-listinvoicerequest(7).
+
+RESOURCES
+---------
+
+Main web site: <https://github.com/ElementsProject/lightning>
+
+[comment]: # ( SHA256STAMP:a862f4bfdcef7db2b7e331ea64f5d79cbdf7553ea5bfd49775a675b21dc7004c)