[llvm-objcopy][docs] Add --update-section to objcopy command guide

[gbreynoo]

Summary

I noticed that when --update-section was added to llvm-objcopy it was not added to the command guide, see 25bcd94234530955c58c6530a9271c813827637c. This change adds it to the docs.

[abrachet]

llvm/docs/CommandGuide/llvm-objcopy.rst

222

I don't think --update-section will add a section if it doesn't exist. It just updates the contents of an existing section. Might also be worth mentioning that it must be the same size or smaller.

[gbreynoo]

Fixed the description after abrachet's comment and updated the help output to mirror the change.

[gbreynoo]

llvm/docs/CommandGuide/llvm-objcopy.rst

222

Good catch @abrachet, I've updated the description.

[jhenderson]

LGTM, but mention the help text fix in the commit message too, please.

llvm/docs/CommandGuide/llvm-objcopy.rst

  trailing whitespace ignored, as is anything following a '#'. Can be specified
  multiple times to read names from multiple files.
 
 .. option:: --strip-unneeded
 
  Remove from the output all local or undefined symbols that are not required by
  relocations. Also remove all debug sections.
 
+.. option:: --update-section <name>=<file>
+
+ Replace the contents of the section ``<name>`` with contents from the file
+ ``<file>``. If the section ``<name>`` is part of a segment, the new contents
+ cannot be larger than the existing section.
+
 .. option:: --version, -V
 
  Display the version of the :program:`llvm-objcopy` executable.
 
 .. option:: --wildcard, -w
 
   Allow wildcard syntax for symbol-related flags. On by default for
   section-related flags. Incompatible with --regex.

llvm/tools/llvm-objcopy/ObjcopyOpts.td

defm add_symbol
     : Eq<"add-symbol", "Add new symbol <name> to .symtab. Accepted flags: "
          "global, local, weak, default, hidden, protected, file, section, object, "
          "function, indirect-function. Accepted but ignored for "
          "compatibility: debug, constructor, warning, indirect, synthetic, "
          "unique-object, before.">,
       MetaVarName<"name=[section:]value[,flags]">;
 
 defm update_section
-    : Eq<"update-section", "Add section <name> with contents from a file <file>.">,
+    : Eq<"update-section", "Replace the contents of section <name> with contents from a file <file>.">,
       MetaVarName<"name=file">;