Make section references easier to search for.

Prefix all section references with the word "section", and avoid line
breaks between "section[s]" and section numbers.

Searching for section references should now be as easy as (for
example):

  grep "section.* 1\.4" dir-spec.txt

Also, while going through section references, lower-case 1 instance of
"Section" to be consistent with the rest and fix 2 broken references.
Make new subsection for routers uploading descriptors.

The rule we're trying to follow here is that sections are for roles
(client, authority, cache, client) and subsections are for operations
performed by the role.  Routers really only perform a single
operation: they periodically upload their descriptors to the
authorities.  That's why there should be a single subsection for this
operation.

This effectively substitutes section number parts "2." with "2.1.":
 * 2 -> 2.1
 * 2.1 -> 2.1.1
 * 2.2 -> 2.1.2
 * 2.2.1 -> 2.1.2.1
 * 2.3 -> 2.1.3
Rename section 3 and remove obsolete overview part.

Section 3 will describe operations and data formats that are related
to the authority role, and section 4 will specify which operations are
performed by caches.  Reflect this in the new section title of section
3, and remove the (already obsolete) overview part listing which
documents are generated by authorities.
Phrase subsection title as operation.
Move section 4.1 "Accepting uploads" to 3.2.

This commit does not yet repair section numbering or references.
Fix references from moving section 4.1 to 3.2.
Rephrase subsection title to be more precise.
Tweak section 3.3 "Microdescriptors".

Phrase subsection title as operation.  Also, remove subsubsection implying
that there would be other subsubsections in the future, which is not the
case.
Move part of 4.1. "Voting" to new 3.4.

This commit does not yet repair section numbering or references.
Fix section references from adding new section 3.4.
Make 3.5 and 3.6 subsections of 3.4.

The vote status document format (3.5) and the process of assigning flags
in a vote (3.6) are both part of the authority operation "exchanging
votes" and not operations on their own.  That's why they should be
subsubsections rather than subsections.

This results in the following changes to section numbers:
 * 3.5 -> 3.4.1
 * 3.6 -> 3.4.2
 * 3.7 -> 3.5
 * 3.8 -> 3.6
 * 3.9 -> 3.7
Add new section 3.5 "Downloading missing certificates".

This subsection is just a placeholder for now.

Also fix section numbers and references.
Copy 4.3 "Downloading router descriptors" after 3.5.

This commit does not yet repair section numbering or references.
Fix section numbers from copying 4.3 after 3.5.
Clean up copied section 3.6 and section 4.3.

Take out cache-only parts from 3.6 and authority-only parts from 4.3.
Copy 4.5 "Downloading extra-info documents" after 3.6

This commit does not yet repair section numbering or references.
Fix section numbers from copying 4.5 after 3.6.
Clean up copied section 3.7 and section 4.5.

Section 3.7 should only contain parts that are relevant for authorities,
and section 4.5 should only contain parts that are relevant for caches
(right now, it also describes what clients should do, but that's something
to change when cleaning up 4.5).
Rename section 3.9.
Rename section 3.10.