Rollback accidental documentation changes

These probably happened during the merge of the commit that made `Copy` opt-in.

Also, convert the last occurence of `/**` to `///` in `src/libstd/num/strconv.rs`
doc: grammar fix
Add doctests for `iter_mut` and `into_iter` of BTreeMap.

Add spacing as dictated by standard rust code style.
Remove references to traits that no longer exist

This specifically means:
- `Deque`
- `Map`
- `Set`
Update sample code to use `getopts::usage` in `print_usage`
Fix an ICE when trying to resolve a struct variant

Unlike a tuple variant constructor which can be called as a function, a
struct variant constructor is not a function, so cannot be called.

If the user tries to assign the constructor to a variable, an ICE
occurs, because there is no way to use it later. So we should stop the
constructor from being used like that.

A similar mechanism already exists for a normal struct, as it prohibits
a struct from being resolved. This commit does the same for a struct
variant.

This commit also includes some changes to the existing tests.

Fixes #19452.
Fix po4a conf file

This line was declared twice, which causes the build of i10n docs to
fail.
Fix inappropriate ## headings

Fixes #15499.
Fix up some {ignore} and {notrust}s

These should be properly annotated instead.

Fixes #16219.
Change default installation directory to %SYSTEMDRIVE%\Rust.
Modify user PATH, rather than system PATH.
Update intro.md
gdb < RUST_BACKTRACE

Fixes #19717
reference: type definition -> type alias

Fixes #9266
Remove mentions of sundown.
Add LLVM ExecutionEngine API
Perform compiler state reset in phase_1
Implement insert for RingBuf
Add doctests for `iter` and `into_iter` for BTreeSet.
rustdoc: Properly inline const items

Build `clean::ConstantItem` values in the `inline` module and
pretty-print the AST for inlined const items.

Doc strings are still missing from inlined constants (see #19773).

Partially address #18156, #19722, #19185

Fix #15821
gedit language spec: add new-style Unicode escapes