Documentation URLs, visible whitespace, and minor touchups

[Mike Taves]

See attached patch, ready to go for trunk.

A summary of changes:

• In general, updated URLs where needed.
• Added some logical spaces, e.g.: "word,word" → "word, word"; typically in "See Also" lists.
• For links to the PostgreSQL Documentation, directing to the "current/static" version (the one without comments).

Specifics:

• performance_tips.xml - updated "checkpoint_segments" section. Removed "checkpoint_segment_size", since I can't even tell what version that was used. Also, I'm not sure what other "many configurations with names starting with checkpoint and WAL" would need to be mentioned here.
• installation.xml
  • the --prefix parameter discussion is best covered in #635, not #160
  • #186 is dead, don't mention it
• reference_editor.xml - Deleted two obsolete references to url="/support/wiki/index.php?plpgsqlfunctions"
• reference_output.xml - ST_AsX3D - there are a few references to "​http://web3d.org/x3d/specifications/ISO-IEC-19776-1.2-X3DEncodings-XML/Part01/EncodingOfNodes.html", which doesn't link as it appears that V1.2 is not available — only V3.0 and up. I didn't modify this, as I'm not sure if a later version is a suitable reference. If so, make it ​http://www.web3d.org/files/specifications/19776-1/V3.2/Part01/EncodingOfNodes.html

Revise as needed.

[pramsey]

On trunk at r9067, thanks!