Update some comments related to naming changes

SDWebImage/SDDiskCache.h

@@ -15,7 +15,7 @@
 // All of these method are called from the same global queue to avoid blocking on main queue and thread-safe problem. But it's also recommend to ensure thread-safe yourself using lock or other ways.
 @required
 /**
- Create a new disk cache based on the specified path.
+ Create a new disk cache based on the specified path. You can check `maxDiskSize` and `maxDiskAge` used for disk cache.
  
  @param cachePath Full path of a directory in which the cache will write data.
  Once initialized you should not read and write to this directory.

SDWebImage/SDDiskCache.m

@@ -172,10 +172,10 @@
     
     // If our remaining disk cache exceeds a configured maximum size, perform a second
     // size-based cleanup pass.  We delete the oldest files first.
-    NSUInteger maxCacheSize = self.config.maxDiskSize;
-    if (maxCacheSize > 0 && currentCacheSize > maxCacheSize) {
+    NSUInteger maxDiskSize = self.config.maxDiskSize;
+    if (maxDiskSize > 0 && currentCacheSize > maxDiskSize) {
         // Target half of our maximum cache size for this cleanup pass.
-        const NSUInteger desiredCacheSize = maxCacheSize / 2;
+        const NSUInteger desiredCacheSize = maxDiskSize / 2;
         
         // Sort the remaining cache files by their last modification time or last access time (oldest first).
         NSArray<NSURL *> *sortedFiles = [cacheFiles keysSortedByValueWithOptions:NSSortConcurrent

SDWebImage/SDImageCache.h

@@ -321,7 +321,7 @@ typedef NS_OPTIONS(NSUInteger, SDImageCacheOptions) {
 #pragma mark - Cache Info
 
 /**
- * Get the size used by the disk cache
+ * Get the total bytes size of images in the disk cache
  */
 - (NSUInteger)totalDiskSize;
 

SDWebImage/SDImageCacheConfig.m

@@ -11,7 +11,7 @@
 #import "SDDiskCache.h"
 
 static SDImageCacheConfig *_defaultCacheConfig;
-static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7; // 1 week
+static const NSInteger kDefaultCacheMaxDiskAge = 60 * 60 * 24 * 7; // 1 week
 
 @implementation SDImageCacheConfig
 
@@ -31,7 +31,7 @@ static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7; // 1 week
         _shouldRemoveExpiredDataWhenEnterBackground = YES;
         _diskCacheReadingOptions = 0;
         _diskCacheWritingOptions = NSDataWritingAtomic;
-        _maxDiskAge = kDefaultCacheMaxCacheAge;
+        _maxDiskAge = kDefaultCacheMaxDiskAge;
         _maxDiskSize = 0;
         _diskCacheExpireType = SDImageCacheConfigExpireTypeModificationDate;
         _memoryCacheClass = [SDMemoryCache class];