lsblk, df: add exclude examples (#4086)

pages/common/df.md

@@ -17,3 +17,7 @@
 - Display statistics on the number of free inodes:
 
 `df -i`
+
+- Display file systems but exclude the specified type:
+
+`df -x {{squashfs}} -x {{tmpfs}}`

pages/linux/lsblk.md

@@ -25,3 +25,7 @@
 - Output info about block-device topology:
 
 `lsblk -t`
+
+- Exclude the devices specified by the comma-separated list of major device numbers:
+
+`lsblk -e {{1,7}}`