stat.2: Improve discussion of 'st_size' for /proc and /sys files

Reported-by: Ursache Vladimir <f35f22fan@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/stat.2

@@ -784,13 +784,21 @@ will generally not trigger automounter action, whereas
 will (but see
 .BR fstatat (2)).
 
-For most files under the
+For many pseudofiles that are autogenerated by the kernel,
+.BR stat ()
+does not return an accurate value in the
+.IR st_size
+field.
+For example, the value 0 is returned for many files under the
 .I /proc
 directory,
-.BR stat ()
-does not return the file size in the
-.I st_size
-field; instead the field is returned with the value 0.
+while various files under
+.IR /sys
+report a size of 4096 bytes, even though the file content is smaller.
+For such files, one should simply try to read as many bytes as possible
+(and append \(aq\e0\(aq to the returned buffer
+if it is to be interpreted as a string).
+.\"
 .SS Timestamp fields
 Older kernels and older standards did not support nanosecond timestamp
 fields.