Use xargs' -P 0 option in JSON style check (CleverRaven#50098)

* use -P 0 option on JSON style check
* xargs' -P 0 option to JSON style doc

Makefile

@@ -1206,7 +1206,7 @@ endif
 style-json: json_blacklist $(JSON_FORMATTER_BIN)
 ifndef CROSS
 	find data gfx -name "*.json" -print0 | grep -v -z -F -f json_blacklist | \
-	  xargs -0 -L 1 $(JSON_FORMATTER_BIN)
+	  xargs -P 0 -0 -L 1 $(JSON_FORMATTER_BIN)
 else
 	@echo Cannot run json formatter in cross compiles.
 endif

doc/JSON_STYLE.md

@@ -49,13 +49,13 @@ The formatting tool can be found alongside the releases as `json_formatter.exe`
 Using `make style-json` will format all files included in the JSON validation test, alternatively:
 ```sh
 # Using git to filter JSON files with uncommitted changes (provided there are no spaces in the file or directory names).
-git diff --name-only '*.json' | xargs -L 1 json_formatter
+git diff --name-only '*.json' | xargs -P 0 -L 1 json_formatter
 
 # Using git to filter modified JSON files in the current branch.
-git diff master --name-only '*.json' | xargs -L 1 json_formatter
+git diff master --name-only '*.json' | xargs -P 0 -L 1 json_formatter
 
 # Per-folder formatting.
-find path/to/desired/folder -name "*.json" -print0 | xargs -0 -L 1 json_formatter
+find path/to/desired/folder -name "*.json" -print0 | xargs -P 0 -0 -L 1 json_formatter
 ```
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