How to shift text left in align environment

Here is a proposal using intertext{} instead of text{}.

documentclass{article}
usepackage{mathtools} % <-- also loads amsmath.

begin{document}

section*{Your Code}
begin{align*}
x1&=x~(text{$1$ is multiplicative identity})
&=x+x~(text{since}~text{ a monosemiring with an identity is idempotent too})
&=x+x1&=x+x^21&=x(1+x)text{i.e}., x1 &=x(1+x)
text{or }1&=1+x~text{by cancellation law}
text{Hence, $R$ is additively absorbing}.
end{align*}

section*{Proposal}
begin{align*}
x1&=x~(text{$1$ is multiplicative identity})
&=x+x~(text{since}~text{ a monosemiring with an identity is idempotent too})
&=x+x1&=x+x^21&=x(1+x)text{i.e}., x1 &=x(1+x)
text{or }1&=1+x~text{by cancellation law}
intertext{Hence, $R$ is additively absorbing}. % <-- intertext
end{align*}

end{document}

Update

This update is motivated by Barbara's (hurtful :)) comment:

This works only by accident. The last line, when included in the alignment, forces the alignment point to the end of that sentence. Removing it from the alignment then allows the remaining lines to be centered as a group, and if the second line were not so long, the group would not move so far to the left. In the present case, just moving the last line entirely out of the alignment/display would work as well.

(Bold formatting is done by Manuel)

I also add (steal) the shortintertext approach that Bernard uses to have a more complete list of approaches.

documentclass{article}
usepackage{mathtools} % <-- also loads amsmath.

begin{document}

section*{Your Code}
begin{align*}
x1&=x~(text{$1$ is multiplicative identity})
&=x+x~(text{since}~text{ a monosemiring with an identity is idempotent too})
&=x+x1&=x+x^21&=x(1+x)text{i.e}., x1 &=x(1+x)
text{or }1&=1+x~text{by cancellation law}
text{Hence, $R$ is additively absorbing}.
end{align*}

section*{Proposal 1}
begin{align*}
x1&=x~(text{$1$ is multiplicative identity})
&=x+x~(text{since}~text{ a monosemiring with an identity is idempotent too})
&=x+x1&=x+x^21&=x(1+x)text{i.e}., x1 &=x(1+x)
text{or }1&=1+x~text{by cancellation law}
intertext{Hence, $R$ is additively absorbing}. % <-- intertext (amsmath package)
end{align*}

section*{Proposal 2 (Barbara Beeton)}
begin{align*}
x1&=x~(text{$1$ is multiplicative identity})
&=x+x~(text{since}~text{ a monosemiring with an identity is idempotent too})
&=x+x1&=x+x^21&=x(1+x)text{i.e}., x1 &=x(1+x)
text{or }1&=1+x~text{by cancellation law}
end{align*}
Hence, $R$ is additively absorbing

section*{Proposal 3 (Bernard)}
begin{align*}
x1&=x~(text{$1$ is multiplicative identity})
&=x+x~(text{since}~text{ a monosemiring with an identity is idempotent too})
&=x+x1&=x+x^21&=x(1+x)text{i.e}., x1 &=x(1+x)
text{or }1&=1+x~text{by cancellation law}
shortintertext{Hence, $R$ is additively absorbing}. % <-- shortintertext (mathtools package)
end{align*}

end{document}