Update files.py (#602)

* Add a bit of documentation on `assume_present` in `file.line` operator

Author: mrshu

pyinfra/operations/files.py

@@ -174,6 +174,7 @@ def line(
     + flags: list of flags to pass to sed when replacing/deleting
     + backup: whether to backup the file (see below)
     + interpolate_variables: whether to interpolate variables in ``replace``
+    + assume_present: whether to assume a matching line already exists in the file
 
     Regex line matching:
         Unless line matches a line (starts with ^, ends $), pyinfra will wrap it such that