Added clarifying examples in Path.xml (dotnet#5382)

Added examples to clarify when a file extension might be limited to three characters (issue dotnet#3579).

Author: shdvr

xml/System.IO/Path.xml

@@ -58,7 +58,7 @@
   
 ## Remarks  
   
- A path is a string that provides the location of a file or directory. A path does not necessarily point to a location on disk; for example, a path might map to a location in memory or on a device. The exact format of a path is determined by the current platform. For example, on some systems, a path can start with a drive or volume letter, while this element is not present in other systems. On some systems, file paths can contain extensions, which indicate the type of information stored in the file. The format of a file name extension is platform-dependent; for example, some systems limit extensions to three characters, and others do not. The current platform also determines the set of characters used to separate the elements of a path, and the set of characters that cannot be used when specifying paths. Because of these differences, the fields of the `Path` class as well as the exact behavior of some members of the `Path` class are platform-dependent.  
+ A path is a string that provides the location of a file or directory. A path does not necessarily point to a location on disk; for example, a path might map to a location in memory or on a device. The exact format of a path is determined by the current platform. For example, on some systems, a path can start with a drive or volume letter, while this element is not present in other systems. On some systems, file paths can contain extensions, which indicate the type of information stored in the file. The format of a file name extension is platform-dependent; for example, some systems limit extensions to three characters (such as FAT16 commonly used on smaller flash storage and older versions of ISO 9660 used on optical media), and others do not. The current platform also determines the set of characters used to separate the elements of a path, and the set of characters that cannot be used when specifying paths. Because of these differences, the fields of the `Path` class as well as the exact behavior of some members of the `Path` class are platform-dependent.  
   
  A path can contain absolute or relative location information. Absolute paths fully specify a location: the file or directory can be uniquely identified regardless of the current location. Relative paths specify a partial location: the current location is used as the starting point when locating a file specified with a relative path. To determine the current directory, call <xref:System.IO.Directory.GetCurrentDirectory%2A?displayProperty=nameWithType>.