[PATCH man-pages v2] rtnetlink.7: Document struct ifa_cacheinfo

[Alex Henrie <alexhenrie24@gmail.com>]

struct ifa_cacheinfo contains the address's creation time, update time,
preferred lifetime remaining, and valid lifetime remaining.

Link: <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/if_addr.h?h=v6.11#n60>
Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
---
Changes from v1:
- Move link to Link line in commit message
- Add the word "remaining" to clarify that the reported values will
  decrease over time
- Say UINT32_MAX instead of -1
- Add a short paragraph to explain the constraints on the minimum and
  maximum lifetimes

Thanks to Kuniyuki and Alejandro for your feedback.
---
 man/man7/rtnetlink.7 | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/man/man7/rtnetlink.7 b/man/man7/rtnetlink.7
index 86ed459bb..ed08834b0 100644
--- a/man/man7/rtnetlink.7
+++ b/man/man7/rtnetlink.7
@@ -176,7 +176,24 @@ IFA_BROADCAST:raw protocol address:broadcast address
 IFA_ANYCAST:raw protocol address:anycast address
 IFA_CACHEINFO:struct ifa_cacheinfo:Address information
 .TE
-.\" FIXME Document struct ifa_cacheinfo
+.IP
+.EX
+struct ifa_cacheinfo {
+    __u32 ifa_prefered; /* Preferred lifetime remaining, in seconds */
+    __u32 ifa_valid;    /* Valid lifetime remaining, in seconds */
+    __u32 cstamp;       /* Creation timestamp, in hundredths of seconds */
+    __u32 tstamp;       /* Update timestamp, in hundredths of seconds */
+};
+.EE
+.IP
+.I ifa_valid
+cannot be zero, and
+.I ifa_prefered
+cannot be greater than
+.IR ifa_valid .
+A value of
+.B UINT32_MAX
+represents an infinite lifetime.
 .TP
 .B RTM_NEWROUTE
 .TQ
-- 
2.47.0