topics: make sure we have some restrictions on topic names This patch makes sure we don't allow topic names with just whitespaces, and other checks which we do for a bookmark.

topic: add test showing no restrictions on topic names Currently we don't have any restrictions on topic names and each of the topic name works. We want them to not work as some of them are used internally and only integers can be rev numbers too.

index: edit pass by native english speaker

topic: reword and simplify explanation of "behind"

topic: tutorial grammar fixes

doc-front-page: link to the changeset evolution section in the document The section is unlikely to be perfect, but at least it exists.

doc-front-page: use a clearer statement about the experimental status This text comes form the Mercurial wiki.

doc-front-pagE: add a paragraph about "why the extension" Let us clarify why there is an extension in the first place.

doc-front-page: expand the paragraph about changeset evolution We clarify the part of the explanation about the changeset evolution concept.

doc-front-page: rename the "next step" section This are pointer to relevant section in the doc. It is a bit double usage with the table of content. We at least clarify the title.