add musl utmpx limitation to usage texts

Signed-off-by: Etienne Cordonnier <ecordonnier@snap.com>

Author: Ecordonnier

src/uu/pinky/pinky.md

@@ -4,4 +4,11 @@
 pinky [OPTION]... [USER]...
 ```
 
-Displays brief user information on Unix-based systems
+Displays brief user information on Unix-based systems.
+
+## Note
+
+When built with musl libc, the `pinky` utility may show incomplete or
+missing user information due to musl's stub implementation of `utmpx`
+functions. This limitation affects the ability to retrieve logged-in
+user details.

src/uu/uptime/uptime.md

@@ -7,3 +7,9 @@ uptime [OPTION]...
 Display the current time, the length of time the system has been up,
 the number of users on the system, and the average number of jobs
 in the run queue over the last 1, 5 and 15 minutes.
+
+## Note
+
+When built with musl libc, the `uptime` utility may show '0 users' due to musl's
+stub implementation of utmpx functions. Boot time and load averages are still
+calculated using alternative mechanisms.

src/uu/users/users.md

@@ -5,3 +5,8 @@ users [FILE]
 ```
 
 Print the user names of users currently logged in to the current host.
+
+## Note
+
+When built with musl libc, the `users` utility may show '0 users' due to musl's
+stub implementation of utmpx functions.

src/uu/who/who.md

@@ -6,3 +6,9 @@ who [OPTION]... [ FILE | ARG1 ARG2 ]
 
 Print information about users who are currently logged in.
 
+## Note
+
+When built with musl libc, the `who` utility may show incomplete or
+missing user information due to musl's stub implementation of `utmpx`
+functions. This limitation affects the ability to retrieve accurate
+details about logged-in users.