tools: lint doc/*.md files

Makefile tasks only lint doc/**/*.md files but omit files that match
doc/*.md. This change adds these previously-omitted files to the list of
files to be linted.

PR-URL: #21361
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Makefile

@@ -1044,7 +1044,7 @@ lint-md-build: tools/remark-cli/node_modules \
 .PHONY: lint-md
 ifneq ("","$(wildcard tools/remark-cli/node_modules/)")
 
-LINT_MD_DOC_FILES = $(shell ls doc/**/*.md)
+LINT_MD_DOC_FILES = $(shell ls doc/*.md doc/**/*.md)
 run-lint-doc-md = tools/remark-cli/cli.js -q -f $(LINT_MD_DOC_FILES)
 # Lint all changed markdown files under doc/
 tools/.docmdlintstamp: $(LINT_MD_DOC_FILES)