PSR1/2/12/Documentation: minor consistency fixes

* Verified that the contents of `<standard>` elements is consistently indented (with four spaces).
* Verified that all `<code>` `title` attributes use proper capitalization and punctuation and are prefixed with "Valid" or "Invalid".

Author: jrfnl

src/Standards/PSR1/Docs/Files/SideEffectsStandard.xml

@@ -1,7 +1,7 @@
 <documentation title="Side Effects">
     <standard>
     <![CDATA[
-    A php file should either contain declarations with no side effects, or should just have logic (including side effects) with no declarations.
+    A PHP file should either contain declarations with no side effects, or should just have logic (including side effects) with no declarations.
     ]]>
     </standard>
     <code_comparison>

src/Standards/PSR1/Docs/Methods/CamelCapsMethodNameStandard.xml

@@ -5,7 +5,7 @@
     ]]>
     </standard>
     <code_comparison>
-        <code title="Valid: method name in camelCase.">
+        <code title="Valid: Method name in camelCase.">
         <![CDATA[
 class Foo
 {
@@ -15,7 +15,7 @@ class Foo
 }
         ]]>
         </code>
-        <code title="Invalid: method name not in camelCase.">
+        <code title="Invalid: Method name not in camelCase.">
         <![CDATA[
 class Foo
 {

src/Standards/PSR12/Docs/Functions/NullableTypeDeclarationStandard.xml

@@ -5,7 +5,7 @@
     ]]>
     </standard>
     <code_comparison>
-        <code title="Valid: no whitespace used.">
+        <code title="Valid: No whitespace used.">
         <![CDATA[
 public function functionName(
     ?string $arg1,
@@ -14,7 +14,7 @@ public function functionName(
 }
         ]]>
         </code>
-        <code title="Invalid: superfluous whitespace used.">
+        <code title="Invalid: Superfluous whitespace used.">
         <![CDATA[
 public function functionName(
     ? string $arg1,
@@ -25,14 +25,14 @@ public function functionName(
         </code>
     </code_comparison>
     <code_comparison>
-        <code title="Valid: no unexpected characters.">
+        <code title="Valid: No unexpected characters.">
         <![CDATA[
 public function foo(?int $arg): ?string
 {
 }
         ]]>
         </code>
-        <code title="Invalid: unexpected characters used.">
+        <code title="Invalid: Unexpected characters used.">
         <![CDATA[
 public function bar(? /* comment */ int $arg): ?
     // nullable for a reason

src/Standards/PSR2/Docs/Methods/MethodDeclarationStandard.xml

@@ -37,7 +37,7 @@ class Foo
 }
         ]]>
         </code>
-        <code title="Invalid: static keyword used before visibility and final used after.">
+        <code title="Invalid: `static` keyword used before visibility and final used after.">
         <![CDATA[
 class Foo
 {