[for-linus][PATCH 17/17] Documentation: bootconfig: Add bootconfig override operator

[Steven Rostedt <rostedt@goodmis.org>]

From: Masami Hiramatsu <mhiramat@kernel.org>

Add a sentence about bootconfig override operator (":=") to
bootconfig.rst.

Link: https://lkml.kernel.org/r/159482884682.126704.7198860675721719878.stgit@devnote2

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 Documentation/admin-guide/bootconfig.rst | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/Documentation/admin-guide/bootconfig.rst b/Documentation/admin-guide/bootconfig.rst
index d6b3b77a4129..a22024f9175e 100644
--- a/Documentation/admin-guide/bootconfig.rst
+++ b/Documentation/admin-guide/bootconfig.rst
@@ -71,6 +71,16 @@ For example,::
  foo = bar, baz
  foo = qux  # !ERROR! we can not re-define same key
 
+If you want to update the value, you must use the override operator
+``:=`` explicitly. For example::
+
+ foo = bar, baz
+ foo := qux
+
+then, the ``qux`` is assigned to ``foo`` key. This is useful for
+overriding the default value by adding (partial) custom bootconfigs
+without parsing the default bootconfig.
+
 If you want to append the value to existing key as an array member,
 you can use ``+=`` operator. For example::
 
@@ -84,6 +94,7 @@ For example, following config is NOT allowed.::
 
  foo = value1
  foo.bar = value2 # !ERROR! subkey "bar" and value "value1" can NOT co-exist
+ foo.bar := value2 # !ERROR! even with the override operator, this is NOT allowed.
 
 
 Comments
-- 
2.26.2