arch: Update doc discouraging use of ArchGetFileName.

Fixes #1704

(Internal change: 2250775)

pxr/base/arch/fileSystem.h

@@ -187,7 +187,10 @@ ArchOpenFile(char const* fileName, char const* mode);
 ARCH_API int64_t ArchGetFileLength(const char* fileName);
 ARCH_API int64_t ArchGetFileLength(FILE *file);
 
-/// Return a filename for this file, if one can be obtained.
+/// Return a filename for this file, if one can be obtained.  Note that there
+/// are many reasons why it may be impossible to obtain a filename, even for an
+/// opened FILE *.  Whenever possible avoid using this function and instead
+/// store the filename for future use.
 ARCH_API std::string ArchGetFileName(FILE *file);
 
 /// Returns true if the data in \c stat struct \p st indicates that the target