refector: add @form-explain-font-size (#15488)

components/form/style/index.less

@@ -10,11 +10,12 @@
 @form-component-max-height: @input-height-lg;
 @form-feedback-icon-size: @font-size-base;
 @form-help-margin-top: (@form-component-height - @form-component-max-height) / 2 + 2px;
+@form-explain-font-size: @font-size-base;
 // Extends additional 1px to fix precision issue.
 // https://github.com/ant-design/ant-design/issues/12803
 // https://github.com/ant-design/ant-design/issues/8220
 @form-explain-precision: 1px;
-@form-explain-height: floor(@font-size-base * @line-height-base);
+@form-explain-height: floor(@form-explain-font-size * @line-height-base);
 
 .@{form-prefix-cls} {
   .reset-component;
@@ -137,6 +138,7 @@ input[type='checkbox'] {
   min-height: @form-explain-height + @form-explain-precision;
   margin-top: @form-help-margin-top;
   color: @text-color-secondary;
+  font-size: @form-explain-font-size;
   line-height: @line-height-base;
   transition: color 0.3s @ease-out; // sync input color transition
 }