minor fixes: discharge of undefined tags warns by default

Author: zjp-CN

text/0000-safety-tags.md

@@ -261,7 +261,7 @@ unsafe fn delegation() {
 }
 ```
 
-Note that discharing a tag that is not defined will raise a hard error.
+Note that discharing a tag that is not defined will raise a warning-by-default lint.
 
 ## Safety Tags are a Part of an Unsafe Function
 
@@ -289,7 +289,7 @@ NOTE:
 * `requires` or `checked` can be specified multiple times, and they will be merged together.
   * Duplicate tags in `requires` will trigger errors.
   * Duplicate tags in `checked` will trigger warning-by-default diagnostics.
-* the scope of a tag is limited to the defined unsafe function, so identical tag name on different
+* the scope of a tag is limited to the defining unsafe function, so identical tag name on different
   unsafe functions won't affect with each other.
 
 ## Auto Generate Safety Docs from Tags