[Docs][llvm-link] Add documentation an CLI options

Currently the documentation on the command line options for llvm-link is
quite sparse. This patch adds in the options that the tool understands
that aren't currently present in the documentation.

Reviewed By: tejohnson, MaskRay

Differential Revision: https://reviews.llvm.org/D155904

llvm/docs/CommandGuide/llvm-link.rst

@@ -39,7 +39,7 @@ OPTIONS
  If specified, :program:`llvm-link` prints a human-readable version of the
  output bitcode file to standard error.
 
-.. option:: -help
+.. option:: --help
 
  Print a summary of command line options.
 
@@ -49,6 +49,59 @@ OPTIONS
  This typically includes a message for each bitcode file linked in and for each
  library found.
 
+.. option:: --override <filename>
+
+  Adds the passed-in file to the link and overrides symbols that have already
+  been declared with the definitions in the file that is passed in. This flag
+  can be specified multiple times to have multiple files act as overrides. If
+  a symbol is declared more than twice, the definition from the file declared
+  last takes precedence.
+
+.. option:: --import <function:filename>
+
+  Specify a function that should be imported from the specified file for
+  linking with ThinLTO. This option can be specified multiple times to import
+  multiple functions.
+
+.. option:: --summary-index <filename>
+
+  Specify the path to a file containing the module summary index with the
+  results of an earlier ThinLTO link. This option is required when 
+  `--import` is used.
+
+.. option:: --internalize
+
+  Internalize the linked symbols.
+
+.. option:: --disable-debug-info-type-map
+
+  Disables the use of a uniquing type map for debug info.
+
+.. option:: --only-needed
+
+  Link only needed symbols.
+
+.. option:: --disable-lazy-loading
+
+  Disable lazy module loading.
+
+.. option:: --suppress-warnings
+
+  Suppress all linker warnings.
+
+.. option:: --preserve-bc-uselistorder
+
+  Preserve the use-list order when writing LLVM bitcode.
+
+.. option:: --preserve-ll-uselistorder
+
+  Preserve the use-list order when writing LLVM assembly.
+
+.. option:: --ignore-non-bitcode
+
+  Do not error out when a non-bitcode file is encountered while processing
+  an archive.
+
 EXIT STATUS
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