Contrast rules: Tweak background text (#2090)

_rules/text-contrast-afw4f7.md

@@ -57,7 +57,7 @@ For each test target, the [highest possible contrast][] between the [foreground
 
 ## Background
 
-Passing this rule does not mean that the text has sufficient color contrast. If all background pixels have a low contrast with all foreground pixels, the success criterion is guaranteed to not be satisfied. When some pixels have sufficient contrast, and others do not, legibility should be considered. There is no clear method for determining legibility, which is why this is out of scope for this rule.
+Passing this rule does not mean that the text has sufficient color contrast. If all background pixels have a low contrast with all foreground pixels, the success criterion is guaranteed to not be satisfied. When some pixels have sufficient contrast, and others do not, legibility should be considered. There is no clear method for determining legibility when some but not all pixels have sufficient contrast, which is why passing this rule does not necessarily mean the corresponding success criterion is met.
 
 This rule is designed specifically for [1.4.3 Contrast (Minimum)][sc143], which has the expected contrast ratio of 4.5:1 (or 3:1 for large text). Because text that fails a contrast ratio of 4.5:1 also fails a contrast ratio of 7:1, this rule maps to [1.4.6 Contrast (Enhanced)][sc146] as well. In order to adequately test the [expectation](#expectation), some of the passed examples do not satisfy [1.4.6 Contrast (Enhanced)][sc146].
 

_rules/text-contrast-enhanced-09o5cg.md

@@ -62,7 +62,7 @@ For each test target, the [highest possible contrast][] between the [foreground
 
 ## Background
 
-Passing this rule does not mean that the text has sufficient color contrast. If all background pixels have a low contrast with all foreground pixels, the success criterion is guaranteed to not be satisfied. When some pixels have sufficient contrast, and others do not, legibility should be considered. There is no clear method for determining legibility, which is why this is out of scope for this rule.
+Passing this rule does not mean that the text has sufficient color contrast. If all background pixels have a low contrast with all foreground pixels, the success criterion is guaranteed to not be satisfied. When some pixels have sufficient contrast, and others do not, legibility should be considered. There is no clear method for determining legibility when some but not all pixels have sufficient contrast, which is why passing this rule does not necessarily mean the corresponding success criterion is met.
 
 When the text color or background color is not specified in the web page, colors from other [origins][] will be used. Testers must ensure colors are not affected by styles from a [user origin][], such as a custom style sheet. Contrast issues caused by specifying the text color but not the background or vice versa, must be tested separately from this rule.