Update no-use-before-declare docs (#3520)

Author: adidahiya

src/rules/noUseBeforeDeclareRule.ts

@@ -25,8 +25,13 @@ export class Rule extends Lint.Rules.TypedRule {
         ruleName: "no-use-before-declare",
         description: "Disallows usage of variables before their declaration.",
         descriptionDetails: Lint.Utils.dedent`
-            This rule is primarily useful when using the \`var\` keyword -
-            the compiler will detect if a \`let\` and \`const\` variable is used before it is declared.`,
+            This rule is primarily useful when using the \`var\` keyword since the compiler will
+            automatically detect if a block-scoped \`let\` and \`const\` variable is used before
+            declaration. Since most modern TypeScript doesn't use \`var\`, this rule is generally
+            discouraged and is kept around for legacy purposes. It is slow to compute, is not
+            enabled in the built-in configuration presets, and should not be used to inform TSLint
+            design decisions.
+        `,
         optionsDescription: "Not configurable.",
         options: null,
         optionExamples: [true],