csplit: clarify how the regex option works (#19157)

Author: waldyrious

pages/linux/csplit.md

@@ -4,22 +4,26 @@
 > This generates files named "xx00", "xx01", and so on.
 > More information: <https://www.gnu.org/software/coreutils/manual/html_node/csplit-invocation.html>.
 
-- Split a file at lines 5 and 23:
+- Split a file in two parts, starting the second one at line 10:
 
-`csplit {{path/to/file}} 5 23`
+`csplit {{path/to/file}} 10`
 
-- Split a file every 5 lines (this will fail if the total number of lines is not divisible by 5):
+- Split a file in three parts, starting the latter parts in lines 7 and 23:
+
+`csplit {{path/to/file}} 7 23`
+
+- Start a new part at every 5th line (will fail if number of lines is not divisible by 5):
 
 `csplit {{path/to/file}} 5 {*}`
 
-- Split a file every 5 lines, ignoring exact-division error:
+- Start a new part at every 5th line, ignoring exact-division error:
 
 `csplit {{[-k|--keep-files]}} {{path/to/file}} 5 {*}`
 
-- Split a file at line 5 and use a custom prefix for the output files:
+- Split a file above line 5 and use a custom prefix for the output files (default is `xx`):
 
 `csplit {{path/to/file}} 5 {{[-f|--prefix]}} {{prefix}}`
 
-- Split a file at a line matching a `regex`:
+- Split a file above the first line matching a `regex` pattern:
 
 `csplit {{path/to/file}} /{{regex}}/`