[libc][docs] Add doc for libc stdio functions

[michaelrj]

Summary

This patch adds a document describing the status of the string functions
in LLVM-libc.

[michaelrj]

update information on the FILE struct.

[michaelrj]

rebase

[lntue]

Shall we mention somewhere, maybe in footnotes or the tables, that file ops are currently only available for linux?

[michaelrj]

add note about FILE only being available on linux, as well as adding the newly added functions.

libc/docs/index.rst

.. toctree::
     implementation_standard
     api_test
     layering
     mechanics_of_public_api
     redirectors
     source_layout
     strings
     runtimes_build
+    stdio

libc/docs/stdio.rst

+=============================
+StdIO Functions in LLVM-libc
+=============================
+
+-------
+Summary
+-------
+
+This document tracks the status of the implementation of stdio functions in LLVM
+Libc.
+
+---------------
+Source location
+---------------
+
+-   The main source for string functions is located at:
+    ``libc/src/stdio`` with subdirectories for internal implementations.
+
+---------------------
+Implementation Status
+---------------------
+
+Formatted Input/Output Functions
+================================
+
+These functions take in format strings and arguments of various types and
+convert either to or from those arguments. These functions are the current focus
+(owner: michaelrj).
+
+=============  =========
+Function_Name  Available
+=============  =========
+\*printf       WIP
+\*scanf
+=============  =========
+
+``FILE`` Access
+===============
+
+These functions are used to interact with the ``FILE`` object type, which is an
+I/O stream, often used to represent a file on the host's hard drive. Currently
+the ``FILE`` object is only available on linux.
+
+=============  =========
+Function_Name  Available
+=============  =========
+fopen          YES
+freopen
+fclose         YES
+fflush         YES
+setbuf
+setvbuf
+ftell
+fgetpos
+fseek          YES
+fsetpos
+rewind
+tmpfile
+clearerr       YES
+feof           YES
+ferror         YES
+flockfile      YES
+funlockfile    YES
+=============  =========
+
+Operations on system files
+==========================
+
+These functions operate on files on the host's system, without using the 
+``FILE`` object type. They only take the name of the file being operated on.
+
+=============  =========
+Function_Name  Available
+=============  =========
+remove
+rename
+tmpnam
+=============  =========
+
+Unformatted ``FILE`` Input/Output Functions
+===========================================
+
+The ``gets`` function was removed in C11 for having no bounds checking and
+therefor being impossible to use safely.
+
+=============  =========
+Function_Name  Available
+=============  =========
+(f)getc
+fgets
+getchar
+fread          YES
+(f)putc
+(f)puts
+putchar
+fwrite         YES
+ungetc
+=============  =========

libc/docs/strings.rst

 
 Primary memory functions
 ========================
 
 .. TODO(gchatelet): add details about the memory functions.
 
 
 =============  =========
-Function_Name  Available
+Function Name  Available
 =============  =========
 bzero          YES
 bcmp           YES
 memcpy         YES
 memset         YES
 memcmp         YES
 memmove        YES
 =============  =========