doc: update os.uptime() and process.uptime() info

PR-URL: https://github.com/nodejs/node/pull/12294
Fixes: https://github.com/nodejs/node/issues/12291
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/os.md

@@ -378,9 +378,8 @@ added: v0.3.3
 
 The `os.uptime()` method returns the system uptime in number of seconds.
 
-*Note*: Within Node.js' internals, this number is represented as a `double`.
-However, fractional seconds are not returned and the value can typically be
-treated as an integer.
+*Note*: On Windows the returned value includes fractions of a second.
+Use `Math.floor()` to get whole seconds.
 
 ## os.userInfo([options])
 <!-- YAML

doc/api/process.md

@@ -1672,6 +1672,9 @@ added: v0.5.0
 The `process.uptime()` method returns the number of seconds the current Node.js
 process has been running.
 
+*Note*: the return value includes fractions of a second. Use `Math.floor()`
+to get whole seconds.
+
 ## process.version
 <!-- YAML
 added: v0.1.3