[PATCH v3 2/2] doc: Document CIDR notation support for -a/--address option

[Jon Maloy <jmaloy@redhat.com>]

Update the man page to document the new CIDR notation support:

- Update -a/--address to show addr[/prefix_len] syntax
- Document that prefix length (0-32) can be appended for IPv4
- Note that mixing CIDR notation with -n results in an error
- Update -n/--netmask to mention CIDR alternative

Signed-off-by: Jon Maloy <jmaloy@redhat.com>
---
 passt.1 | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/passt.1 b/passt.1
index db0d662..117239f 100644
--- a/passt.1
+++ b/passt.1
@@ -156,10 +156,14 @@ By default, the advertised MTU is 65520 bytes, that is, the maximum 802.3 MTU
 minus the length of a 802.3 header, rounded to 32 bits (IPv4 words).
 
 .TP
-.BR \-a ", " \-\-address " " \fIaddr
+.BR \-a ", " \-\-address " " \fIaddr\fR[\fB/\fR\fIprefix_len\fR]
 Assign IPv4 \fIaddr\fR via DHCP (\fByiaddr\fR), or \fIaddr\fR via DHCPv6 (option
 5) and an \fIaddr\fR-based prefix via NDP Router Advertisement (option type 3)
 for an IPv6 \fIaddr\fR.
+For IPv4 addresses, an optional \fB/\fR\fIprefix_len\fR (0-32) can be
+appended in CIDR notation (e.g., 192.168.1.1/24). This is an alternative to
+using the \fB-n\fR, \fB--netmask\fR option. Mixing CIDR notation with
+\fB-n\fR results in an error.
 This option can be specified zero (for defaults) to two times (once for IPv4,
 once for IPv6).
 By default, assigned IPv4 and IPv6 addresses are taken from the host interfaces
@@ -172,7 +176,9 @@ is assigned for IPv4, and no additional address will be assigned for IPv6.
 .TP
 .BR \-n ", " \-\-netmask " " \fImask
 Assign IPv4 netmask \fImask\fR, expressed as dot-decimal or number of bits, via
-DHCP (option 1).
+DHCP (option 1). Alternatively, the prefix length can be specified using CIDR
+notation with the \fB-a\fR, \fB--address\fR option (e.g., \fB-a\fR 192.168.1.1/24).
+Mixing \fB-n\fR with CIDR notation results in an error.
 By default, the netmask associated to the host address matching the assigned one
 is used. If there's no matching address on the host, the netmask is determined
 according to the CIDR block of the assigned address (RFC 4632).
-- 
2.52.0