Issue: SPR-16547

This commit restructures the existing content to separate out Overview
content into its own section, and also introduces more formal structure
where reference type content can be expanded over time.

Issue: SPR-16547

Issue: SPR-16572

Issue: SPR-16571

Prior to this commit, the `HttpEntityMethodProcessor` would check HTTP
conditions on non-safe requests (i.e. not GET/HEAD). This would prevent
Controllers from returning `ResponseEntity` containing response headers
with updated values of `"Last-Modified"` or `ETag` once the resource has
been updated.

This commit avoids those checks for non GET/HEAD requests - this code
can still be leveraged from Controllers themselves using
`ServletWebRequest::checkNotModified` methods.

Issue: SPR-15780

Issue: SPR-16570

Issue: SPR-16568

Issue: SPR-16494

After send error, ignore calls to complete or completeWithError by the
application in the same thread (e.g. try-catch block), to avoid a
competing with an error callback from servlet container during which
the same action is taken.

Issue: SPR-16548

Issue: SPR-16548

When checking whether there is still request body the first method
that should be checked is ServletInputStream.isReady() and then
ServletInputStream.isFinished(). ServletInputStream.isReady() is the active
method whereas the ServletInputStream.isFinished() is not.
It is important to call ServletInputStream.isReady() because if it returns
false it will schedule a dispatch and if the request body is already read it will
send onAllDataRead event.

Issue: SPR-16521

Issue: SPR-16107

(cherry picked from commit b70d400c)

MockRestRequestMatchers was using org.junit.Assert.assertNotNull
and thus could not be used in projects that use e.g. TestNG instead
of JUnit 4.

EncoderHttpMessageWriter checks explicitly for Mono publishers and sets
the content length, if it is known for the given data item.

Issue: SPR-16542