Update editorconfig and the documented C-style for sign.c/sound.c

closes: #17786

Signed-off-by: Damien Lejay <[email protected]>
Signed-off-by: Christian Brabandt <[email protected]>

Author: dlejay

.editorconfig

@@ -35,3 +35,12 @@ trim_trailing_whitespace = false
 [src/testdir/test*.vim]
 # Some tests need trailing whitespaces, for example `set showbreak=>>\ `
 trim_trailing_whitespace = false
+
+#
+# Some custom files do not use tabs
+#
+[src/sign.c]
+indent_style = space
+
+[src/sound.c]
+indent_style = space

runtime/doc/develop.txt

@@ -1,4 +1,4 @@
-*develop.txt*   For Vim version 9.1.  Last change: 2025 May 05
+*develop.txt*   For Vim version 9.1.  Last change: 2025 Jul 18
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -237,6 +237,10 @@ you don't need to do anything due to the |modeline|.
 
 For other editors an `.editorconfig` is provided at the root of the repo.
 
+For the source files `sign.c` and `sound.c` and any new file use only spaces,
+no tabs. In addition, any new file must include a modeline with `set et` to
+pass the indentation test.
+
 
 DECLARATIONS						*style-declarations*