doc: grammar fixes

src/doc/complement-design-faq.md

 % The Rust Design FAQ
 
-This document describes decisions were arrived at after lengthy discussion and
+This document describes decisions that were arrived at after lengthy discussion and
 experimenting with alternatives. Please do not propose reversing them unless
 you have a new, extremely compelling argument. Note that this document
 specifically talks about the *language* and not any library or implementation.

src/doc/guide-unsafe.md

@@ -446,7 +446,7 @@ possible in two ways: the `#[start]` attribute, or overriding the
 default shim for the C `main` function with your own.
 
 The function marked `#[start]` is passed the command line parameters
-in the same format as a C:
+in the same format as C:
 
 ```
 #![no_std]
@@ -593,7 +593,7 @@ standard library itself.
 # Interacting with the compiler internals
 
 > **Note**: this section is specific to the `rustc` compiler; these
-> parts of the language may never be full specified and so details may
+> parts of the language may never be fully specified and so details may
 > differ wildly between implementations (and even versions of `rustc`
 > itself).
 >

src/libserialize/json.rs

@@ -106,7 +106,7 @@ fn main() {
 
 ## Using the `ToJson` trait
 
-The examples above use the `ToJson` trait to generate the JSON string, which required
+The examples above use the `ToJson` trait to generate the JSON string, which is required
 for custom mappings.
 
 ### Simple example of `ToJson` usage