Update the code documentation

Author: dreampiggy

SDWebImagePDFCoder/Classes/SDImagePDFCoder.h

@@ -13,16 +13,16 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+static const SDImageFormat SDImageFormatPDF = 13;
+
 /***
  SDImagePDFCoder is a PDF image coder, which use the built-in UIKit/AppKit method to decode PDF images. And it will use the Core Graphics to draw PDF images when needed (For example, current firmware does not support built-in rendering). This class does not use PDFKit framwork because we focus on PDF vector images but not document pages rendering.
  
- @note: For iOS/tvOS, from iOS/tvOS 11+, Apple support built-in vector scale for PDF image in `UIImage`. Which means you can just get set a image to the `UIImaegView`, then changing image view's bounds, contentMode to adjust the PDF image without losing any detail. However, when running on lower firmware, we don't support vector scaling and parse PDF image as a bitmap image. You can use `SDWebImageContextPDFImageSize` and `SDWebImageContextPDFImagePreserveAspectRatio` during image loading to specify a desired size (such as larger size for view rendering).
- @note: For macOS, Apple support built-in vector scale for PDF image in `NSImage`. However, `NSImage` is mutable, you can change the size after image was loaded.
- @note: By default we parse the first page (pageNumber = 1) of PDF image, for custom page number, check `SDWebImageContextPDFPageNumber` context option.
+ @note For iOS/tvOS, from iOS/tvOS 11+, Apple support built-in vector scale for PDF image in `UIImage`. Which means you can just get set a image to the `UIImaegView`, then changing image view's bounds, contentMode to adjust the PDF image without losing any detail. However, when running on lower firmware, we don't support vector scaling and parse PDF image as a bitmap image. You can use `SDImageCoderDecodeThumnailPixelSize` and `SDWebImageContextImagePreserveAspectRatio` during image loading to specify a desired size (such as larger size for view rendering).
+ @note For macOS, Apple support built-in vector scale for PDF image in `NSImage`. However, `NSImage` is mutable, you can change the size after image was loaded.
+ @note By default we parse the first page (pageNumber = 1) of PDF image, for custom page number, check `SDWebImageContextPDFPageNumber` context option.
+ @note If you call the coder directly, use the coder option (See `SDWebImagePDFCoderDefine.h`) instead of the context option.
  */
-
-static const SDImageFormat SDImageFormatPDF = 13;
-
 @interface SDImagePDFCoder : NSObject <SDImageCoder>
 
 @property (nonatomic, class, readonly) SDImagePDFCoder *sharedCoder;