Fix three rustdoc::broken_intra_doc_links

[Enselic]

I am working on a bit on rust-lang/rust#93588 and found these warnings distracting, so I decided to help fix them.

Step-by-step:

1. cargo +nightly doc --manifest-path diesel/Cargo.toml

Expected result:

No warnings

Actual result:

warning: unresolved link to `alias`
  --> diesel/src/query_source/aliasing/alias.rs:16:10
   |
16 | /// See [alias!] for more details.
   |          ^^^^^^ no item named `alias` in scope
   |
   = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default
   = note: `macro_rules` named `alias` exists in this crate, but it is not in scope at this link's location

warning: unresolved link to `alias`
  --> diesel/src/query_source/aliasing/aliased_field.rs:19:10
   |
19 | /// See [alias!] for more details.
   |          ^^^^^^ no item named `alias` in scope
   |
   = note: `macro_rules` named `alias` exists in this crate, but it is not in scope at this link's location

warning: unresolved link to `allow_tables_to_appear_in_same_query`
    --> diesel/src/macros/mod.rs:1132:28
     |
1132 | /// still need to invoke [`allow_tables_to_appear_in_same_query!`] for these two tables to
     |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `allow_tables_to_appear_in_same_query` in scope
     |
     = note: `macro_rules` named `allow_tables_to_appear_in_same_query` exists in this crate, but it is not in scope at this link's location

[JohnTitor]

Thanks! Looks good.