Mojo docs: improve visibility of MinVersion note

Prior to this CL, the note that MinVersion required non-primitive types
to be nullable in structs was after the discussion of ordinality for
struct fields. After this CL, that requirement is mentioned near the
beginning of the discussion about struct field versioning to make it
more discoverable.

I also did a few drive-by typo fixes.

Bug: N/A
Change-Id: Iad906ab2d1278f5855590f18c1aff4c394586852
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2881026
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#880685}

mojo/public/tools/bindings/README.md

@@ -320,7 +320,7 @@ enum definition. By default, values are based at zero and increment by
 1 sequentially.
 
 The effect of nested definitions on generated bindings varies depending on the
-target language. See [documentation for individual target languages](#Generated-Code-For-Target-Languages)
+target language. See [documentation for individual target languages](#Generated-Code-For-Target-Languages).
 
 ### Constants
 
@@ -346,7 +346,7 @@ struct Employee {
 ```
 
 The effect of nested definitions on generated bindings varies depending on the
-target language. See [documentation for individual target languages](#Generated-Code-For-Target-Languages)
+target language. See [documentation for individual target languages](#Generated-Code-For-Target-Languages).
 
 ### Interfaces
 
@@ -579,7 +579,8 @@ struct Employee {
 };
 ```
 
-and you would like to add birthday and nickname fields. You can do:
+and you would like to add birthday and nickname fields. You can add them as
+optional types with a `MinVersion` like so:
 
 ``` cpp
 struct Employee {
@@ -590,6 +591,12 @@ struct Employee {
 };
 ```
 
+*** note
+**NOTE:** Mojo object or handle types added with a `MinVersion` **MUST** be
+optional (nullable). See [Primitive Types](#Primitive-Types) for details on
+nullable values.
+***
+
 By default, fields belong to version 0. New fields must be appended to the
 struct definition (*i.e*., existing fields must not change **ordinal value**)
 with the `MinVersion` attribute set to a number greater than any previous
@@ -635,11 +642,6 @@ struct Employee {
 };
 ```
 
-*** note
-**NOTE:** Newly added fields of Mojo object or handle types MUST be nullable.
-See [Primitive Types](#Primitive-Types).
-***
-
 ### Versioned Interfaces
 
 There are two dimensions on which an interface can be extended