Better document File.Move across disk volumes. (dotnet#5249)

* Better cover File.Move across disk volumes.

Currently the documentation claims that IOException is only thrown if `destFileName` exists. This is not true - moving across disk volumes involves copying and opens various IOException possibilities, e.g. "no space left on device".

* Apply suggestions from code review

Co-authored-by: Konstantin Gukov <[email protected]>
Co-authored-by: Adam Sitnik <[email protected]>

Author: 3 people

xml/System.IO/File.xml

@@ -2518,6 +2518,8 @@ Note that if you attempt to replace a file by moving a file of the same name int
 
 The `sourceFileName` and `destFileName` arguments can include relative or absolute path information. Relative path information is interpreted as relative to the current working directory. To obtain the current working directory, see <xref:System.IO.Directory.GetCurrentDirectory%2A>.
 
+Moving the file across disk volumes is equivalent to copying the file and deleting it from the source if the copying was successful.
+
 If you try to move a file across disk volumes and that file is in use, the file is copied to the destination, but it is not deleted from the source.
 
 For a list of common I/O tasks, see [Common I/O Tasks](/dotnet/standard/io/common-i-o-tasks).
@@ -2534,6 +2536,10 @@ The following example moves a file.
         </remarks>
         <exception cref="T:System.IO.IOException">
           <paramref name="destFileName" /> already exists.
+
+          -or-
+
+          An I/O error has occurred, e.g. while copying the file across disk volumes.
         </exception>
         <exception cref="T:System.IO.FileNotFoundException">
           <paramref name="sourceFileName" /> was not found.
@@ -2595,6 +2601,8 @@ This method works across disk volumes, and it does not throw an exception if the
 
 The `sourceFileName` and `destFileName` arguments can include relative or absolute path information. Relative path information is interpreted as relative to the current working directory. To obtain the current working directory, see <xref:System.IO.Directory.GetCurrentDirectory%2A>.
 
+Moving the file across disk volumes is equivalent to copying the file and deleting it from the source if the copying was successful.
+
 If you try to move a file across disk volumes and that file is in use, the file is copied to the destination, but it is not deleted from the source.
 
 For a list of common I/O tasks, see [Common I/O Tasks](/dotnet/standard/io/common-i-o-tasks).
@@ -2611,6 +2619,10 @@ The following example moves a file.
         </remarks>
         <exception cref="T:System.IO.IOException">
           <paramref name="destFileName" /> already exists and <paramref name="overwrite" /> is <see langword="false" />.
+
+          -or-
+
+          An I/O error has occurred, e.g. while copying the file across disk volumes.
         </exception>
         <exception cref="T:System.IO.FileNotFoundException">
           <paramref name="sourceFileName" /> was not found.</exception>